Howdy,
 
I have Sonar X3 Studio and I'm using XLN Audio's Addictive Keys with my Roland FP4.
 
Having some issues currently with sustaining, sometimes it works, sometimes it doesn't... I can't properly replicate the issue each time, but a good 70% of the time I record the sustain simply isn't there when I playback the track.
 
I have the m-audio mtrack, using Windows 10. I listen through my m-track out, so I hear the addictive keys and not my piano speakers- the sustain is fine while playing. It just seems to be broken on playback.
 
 
Any advice would be muchly appreciated as I'm at my wits-end.
 
thanks,
enjoi
 

Any chance you are filtering out midi events (like sustain)?

There is a problem in X3 when the CC64 value is 0. I do not know if this is your issue. It is easy to see if it is by looking at the Event Viewer. There was a pretty lengthy thread about it and the details regarding the bug are here. There is no fix to the program but I wrote a script to fix the MIDI track and posted it in the thread here.

The issue with CC64=0 (pedal up) not being processed correctly meant that notes sustained when they should not. The OP's description seems to be the opposite: "sustain simply isn't there when I playback the track".
 
The first thing to check would be that the pedal events are being recorded in the first place. They should be visible in all views - triangles in clips in the Track View, a CC64 controller lane in the PRV, and CC64 events in the Event List view. If the events aren't being recorded, check that recording of Controllers is enabled under MIDI > Playback and Recording in Preferences.
 
If the controller events are present but not being properly rendered by the synth, make sure pedal ups are CC64=0 (or something less than 64), and pedal downs are CC64 =127 (or something greater 63). Possibly the keyboard's response to polarity of the pedal is getting reversed; this can happen if the pedal is depressed while the keyboard is powered up.