Why do notes fade out in the middle of a score and fade back in during playback?

I'm sorry. I'm having two more issues that I didn't address in my last post. This doesn't happen with every score, but in some scores when I try to play the score (this is usually when different staffs have different velocity levels, usually after importing a MIDI) some notes, normally a whole note, will fade out to silence and get louder again on another measure after that. I've tried resetting the velocity for everything and using dynamics but it still does it even after exporting it to .WAV. As I said before, it doesn't happen with every score, but it's irritating nonetheless.

My second problem isn't too bad, but I wish I could fix it.

I only use the sf2 that comes with Musescore, but I recently installed Sonatina Symphonic orchestra (the sfz version) because I heard how good it was. I love it but there's one problem, the volume. The volume of the samples is incredibly low compared to other soundfonts (especially Fluid) I'll use six instruments of the same instrument and use the same notation, turn it all the way up in the mixer and I can still barely hear it. It's so low compared to the other soundfont that fluid drowns out Sonatina.