This is kind of strange, but no matter which bass plugin I use some notes will not play.
 
I downloaded the trial version of Modobass. Really cool, but the same problem, it will not play certain notes. I just now downloaded a freebie called Ample Bass. Sounds pretty good and will play bass from tabs, such as Guitar Pro tabs. The app comes with quite a few sample tabs, but even with the provided tabs it will not play certain notes.
 
Example of that is, it will play open notes on the low E but will not play the fretted notes on say, the 5th or 7th fret D string. Doesn't matter if it is tabs, midi, standalone or in the DAW.
 
Why will the plugins not play these notes? I can see the dots in the proper place as the notes are played, just no sound.

Funny you mention this as I recently purchased Modo , and did some work for a guy who used music notation through Sibelus to compose a bass line and he exported the notation in the form of a midi file and asked me to produce audio using the midi file.
 
In order for the full bass line to play correctly, I had to transpose all the midi notes up one octave.  So your issue may be as simple as a creating a midi map. (that means notation software might have a C note on the 2ND octave but Modo's midi mapping places the C note at first octave)
 
 

Another possibility is Modo Bass has two fretting styles , one plays based on first position fretting technique, and another uses a neck climbing style. If you read the manual you could toggle between these see if there is a difference.

You mention seeing the dots but no sound. You are talking about the fretboard dots displayed in Modo Bass? Or do you mean tablature , notation dots?

Could this be a latency issue, very low latency settings on your audio interface can cause dropped notes.

Do other VST instruments have similar issues?

Does the problem occur on both legato and staccato notes? On some instruments playing legato will trigger hammer on articulations. It should still sound though.

Are the demo versions limited in some way, often they will be limited in range, the number of articulations or will inject a burst of noise or a short silence.

When I play the Ample Bass from my keyboard, I can hit every note from E1 (open E string) up to F4 (highest fret on G string).
 
The only issue I have run into with Bass plugins is when some of the MIDI notes in an imported pattern are in the octave below the instruments lowest note.
 
When I see this happening, it's time to open up the piano roll and fiddle around, or just transpose the whole pattern up one octave.
 
If this has already been ruled out, then I would either suspect latency issues, or maybe a need to bump up your MIDI playback buffers to 500 (from the default 250).  I have heard that this is a more common issue with dense drum tracks.

StuH and Glyn Barnes, Thanks for the replies. I only have the trial version of modo, I'm sure it's expired now, though and I'm not currently using it.
 
It doesn't matter which Bass VST I use, they ALL do the same thing, but I noticed last night using the same midi file that each VST will play it differently. The one included in Sonar, SI-Bass, will not play the open low E notes.
 
This doesn't happen with other instruments like Drums or piano, just bass and doesn't appear to be a latency issue.
 
The plugin I'm using now is called Ample Bass. It was just a freebie I grabbed from Reverb. It's not bad and sounds pretty good. The dots I referred to represent the string/note being played in this vst.
 
It's just odd. I noticed the keyboard at the bottom will play the open string to the 4rd fret.
 
And it doesn;t matter if it is the stand-alone app or in the DAW.

abacab
When I play the Ample Bass from my keyboard, I can hit every note from E1 (open E string) up to F4 (highest fret on G string).
 
The only issue I have run into with Bass plugins is when some of the MIDI notes in an imported pattern are in the octave below the instruments lowest note.
 
When I see this happening, it's time to open up the piano roll and fiddle around, or just transpose the whole pattern up one octave.
 
If this has already been ruled out, then I would either suspect latency issues, or maybe a need to bump up your MIDI playback buffers to 500 (from the default 250).  I have heard that this is a more common issue with dense drum tracks.

How do you transpose midi up an octave? You and StuH both mentioned this and I was thinking there is probably something in the midi causing it so this may be the problem.

Bflat5
 
How do you transpose midi up an octave? You and StuH both mentioned this and I was thinking there is probably something in the midi causing it so this may be the problem.

If you double click on the MIDI clip it will open up the piano roll view.  That will show you where the notes are in relation to the keys on the keyboard at the left side.  Click the keys with the mouse and go all the way down until you can't make any more sound.  Are all the notes in the pattern above that key?  If not you can move them manually.
 
Or you can transpose the whole clip.  Highlight the clip (select) in track view, then go to menu Process > Transpose.  If you dial in +12 and hit OK, you will go up 12 semitones (one octave).

abacab

Bflat5
 
How do you transpose midi up an octave? You and StuH both mentioned this and I was thinking there is probably something in the midi causing it so this may be the problem.

If you double click on the MIDI clip it will open up the piano roll view.  That will show you where the notes are in relation to the keys on the keyboard at the left side.  Click the keys with the mouse and go all the way down until you can't make any more sound.  Are all the notes in the pattern above that key?  If not you can move them manually.
 
Or you can transpose the whole clip.  Highlight the clip (select) in track view, then go to menu Process > Transpose.  If you dial in +12 and hit OK, you will go up 12 semitones (one octave).

Thanks, I'll give that a try right now.
 
 
EDIT: Looks like that fixed it! Learned something new, too!  Thanks for the replies and help.