Could be MIDI cc7 (volume) and cc11 (expression).
With these types of libraries, you should typically always set these two (really, you should WORK them) in every track / patch that uses them.
I've seen various points of view about this, but once you know and understand, I think it's really quite simple.
There is a preverences option - something like "Zero all controllers on Stop".
A lot of people seem to "uncheck" (turn off) this option ... which is NOT the best at all.
There is another option ... something like "Search back controlller on Start".
Both those options should be turned ON.
That way, when you use MIDI CC to control Vol/Expression (or anything else), when you press Start / Play the last value used in the MIDI track will be fetched, so the following note will be played under the influence of the intended CC values.
You must however, then always remember, to actually set a MIDI CC for Vol/Expression at the start of every track ... which you should do anyway.
Good luck :-)
Cheers - Cliff