I’ve been having the same issue and I might have a partial answer for you. There’s these two yellow meters to the left of the piano roll.
If you haven’t adjusted those I would change that. The one on the left changes the tune and I’m not sure exactly what the one on the right does but it seems to change volume and possibly add vibrato/reverb or something (I tried it on some marimbas once and it was weird).
You can also change the level knob which is supposed to change the gain but I find the meter to the left of the piano roll is still necessary for that to have any effect.
My current issue is that that meter resets every time I load up reaper. I’ve made a post here, which no one has responded to as of yet. I think I have to mess with the CC settings but I can’t figure out how to do that because nothing happens when I try to right click on it. I might try to learn more about automation to figure this out, if someone happens to come across this and can help me out too that would be much appreciated. Anyways, sorry that this is the best answer I have, hope it helps.