Midi notes below certain velocity do not play

That's very unusual.  I don't have any experience with NI's B4.  But I know real ones aren't velocity sensitive...  Does it have some form of velocity settings somewhere.  And if so, when you change them, does the behavior change?

M-Audio FastTrack Pro is the sound card
(I think it is not sound card related, on another track b4 played all notes fine regardless of velocity)
so I think it is something "in the track properties" or "clip properties"

This is a Native Instruments vst? Other people have had problems with MIDI notes not playing. I had a problem with their Drummer instruments not playing certain hits. 

Does the answer in this thread help? It's based on Studio Drummer/Kontakt 5 though.

http://forum.cakewalk.com/tm.aspx?m=2635143&mpage=1

Right click on a MIDI note that doesn't make any sound. There should be a box saying "Channel" and this might say something entirely different than "1". If it does, then this is most likely why you don't hear any sound when it triggers. When you record MIDI, the MIDI chanel on your controler is apparently inserted into the note data and will playback that particular note on channel 2 - or whatever it's set to. Some VSTs will play back that note on a different channel than what the loaded instrument is geting it's input from. This way you can load one patch for kick and snare (channel 1). one patch for hats and toms (channel 2) and the last one for cymbals (channel 3) in Kontakt for example, and just use one MIDI track to control the three different channels. However, I believe this doesn't apply to Session Drummer so it just play back all the MIDI that is sent to it. I think it's a bit of old school MIDI. IDK. 

so check the silent notes to see that they are sending to channel one (or whatever channel the notes you can hear) and you should be able to play back all the notes. 

You can highlight all the silent notes and change the channel in the Event Inspector if I recall correctly. 

Under preferences make sure Zero Controllers on stop is unchecked.
If you used any pedals or controllers you may have set one of the settings lower and it has remained at that setting.  If your Echo on is still on it may be picking up a controller signal from your keyboard and having and effect.

On the B4 under the organ tab there is a velocity setting in the upper left section under Organ.  It kind of works in reverse,  as you set it higher it gets quieter. Set it to 0.  Also check all the main volume and swell knobs/sliders to make sure none of them are turned down.

In PRV check in the Controller pain for any CC's that may have been recorded and adjust to see if that solves the problem.