I'm at my wit's end. I have gotten nearly every other instrument to work but timpani. I don't know what else to try. I'm using Finale 2006 with the full version of GPO. I've had this same trouble with some of the other instruments but have worked around it by wiggling the mod wheel or opening an existing file that already worked and adding or subtracting new instruments. I've never gotten timpani to sound more than a very distant muffled rumble. For some odd reason, two of my arrangements in Finale have had all the instruments speaking clearly. When I start any new project with the wizard, most of the instruments barely speak, even when everything is set up the way these two arrangements that work are set up. Therefore, now when I set up a new arrangement, I just open one of the ones that work and add or subtract instruments. It works like a charm except for timpani and some of the other percussion sounds. They are very skittish. How do I get the timpani, bass drum, and other similar sounds to work?
Since you kindly sent me your file privately, I was able to examine it. As it was a Windows file, I re-set it up on my mac with:
GPO slot 1, ch1...5: Oboe, bassoon, Trp1, trp2, Timpani KS.
GPO slot 2, ch17...: Vl1 lush, Vl2 lush, Viola lush, Cello lush, harpsichord.
Ok. To me your problem simply comes from the fact that there is no written dynamics whatsoever in your file. For the whole score, the dynamic for every instrument is "64" (from Playback Control's default velocity) which roughly corresponds to "mf". It is true that GPO's timpani is rather discrete in p-mf range, but certainly there is no reason to keep mf for the timpani the whole score long in such joyful a piece!
Simply add a few dynamics here and there (and not only to the timpani!) and you will get a better balance.
Adding "f" dynamic to the timpani part immediately made the trick for me.
For a better notation, you should also clean up your staff grouppings. It is pretty messy for now!