I use drum samples from a library I bought. And I put each drum instrument (or group, like Toms, Rides, Crashes) in different MIDI channels in my DAW (Cubase Pro 11) so I can quickly make the Volume balance between them.
OK. Bought sample sets usually are not too low in volume. To the contrary, they are most of the time as loud as póssible. I had this idea that the volume of the samples could be very low, and therefore GrooveAgent couldn’t squeeze them to be loud enough? Just a wild guess, of course?
Another wild guess is the master volume of GrooveAgent. It’s a little horizontal fader at the top. Maybe you poked around with that and forgot about it? I’m not sure what impact that has on the overall output of GrooveAgent but it’s worth a try! 
That master volume slider is tiny on my 24" monitors and poorly marked. I am sure someone will disagree and I don’t care. I had to read through all that nonsense(along with some honest attempts) which, as often happens, fails to address the actual question, to get to the only response that made any sense. Almost cursed Google again for bothering me with another non-answer. Thanks, HowlingUlf!