I have noticed a strange behavior on one project.  I have 5 instruments running through one instance of Aria, so that I can use the Garritan Jazz and Big Band sounds.  The output for each of these instruments goes to its own audio track.  I noticed that about 20% of my bass notes are not playing.  I don't notice this on any of the other 4 Aria tracks.
 
If I freeze the track, all the notes are played correctly.  That makes me wonder if there is something going on with Aria dropping notes if the CPU is too busy.
 
I thought that I might have a problem of MIDI notes overlapping, but I ran the SINGLE script which is supposed to fix that.
 
Has anybody see behavior like this?
 
I have not tried moving the bass to its own instance of Aria.  That's probably a good test to try.

The most common cause of MIDI notes not rendering in real time is having the MIDI Prepare Using buffer set to low. Try the old default of 500ms. The new default of 250 is too small for many systems.

Another possibility is that you have Aria’s polyphony set to low for the number of notes that’s playing. Raising Aria’s polyphony will help if this is the case.

I've bumped into both those situations... GPO4 seems to be a pretty popular product.

MarioD
Another possibility is that you have Aria’s polyphony set to low for the number of notes that’s playing. Raising Aria’s polyphony will help if this is the case.

I don't see anywhere to set the polyphony on Aria.  But I can play chords on the bass, so I don't think that is the problem.
 
I switched the bass over to SI-Basses and it has the same problem.  It is not consistent.  It regularly drops about 25% of the notes, but it isn't the same notes each time.  And I really don't hear it doing that on my other instruments.  Maybe it is happening on all of them and I am only hearing it because the bass line is so apparent.
 
It seems like a CPU-saturation thing.  My project currently has
4 Aria tracks
5 SessionDrummer tracks
1 recorded track
1 SI-basses track
 
If I render everything on the fly, I lose about 25 of the bass notes.
If I freeze Aria, I only lose about 10% of the bass notes.
If I freeze both Aria and SessionDrummer, I get almost all of the bass notes
If I freeze SI-basses (regardless of whether the other synths are frozen) I get all of the bass notes.
 
I've been thinking about upgrading this machine.  I may have to bite the bullet.

I have had this issue with other bass instruments and sometimes drums, so there is a good chance it is not Aria.  Since I changed buffer values like brundlefly is saying, I have rarely had this. 
 
When I have this problem, one of the following fixes it:1-Hit the MIDI button on the Control Bar to clear all MIDI messages.  2-Freeze on of the MIDI soft synths.  3-Restart Sonar.  I obviously start with #1 since it is the least annoying.
 
There is a related bug I have found that in some projects: more than x number of soft synths will make one of them stop playing.  This is not a memory issue as I see it happen when my memory is not even close to be used up.  x can be 5 or 9 or some other number.  The only solution for this is to freeze one.  This does not sound like the case here.  This issue sounds like buffers.

Just cange your MIDI buffers and it will fix your issue. No need to change it to poly

brundlefly
The most common cause of MIDI notes not rendering in real time is having the MIDI Prepare Using buffer set to low. Try the old default of 500ms. The new default of 250 is too small for many systems.

That seems to help.

I’m glad you have things working properly.
 
FYI – to set the polyphony in Aria click on settings (that’s the last option on the right hand side of the main screen) then click on the number to the immediate right of Inst.Poly. You will then see the polyphony options that you can choose.