Are you using 5.0 or 5.1 (it makes a big difference)?
If you are on 5.0, I recommend that you update immediately (but expect a few carefully-made adjustments to your existing scores to change — there are some changes in priority for playback).
With 5.1 you have a radio button in File > Preferences > Playback > Display which allows you to see the library names instead of SoundIds in the Mixer. So you can choose 'GPO Basic Percussion' or whatever instead of the rather flaky and pretty unhelpful SoundIds.
Then you can click on a percussion stave, and use House Styles > Edit Instruments to have a look at how the staff is allocated. Edit Staff Type lets you see (and edit) which sounds belong on which staff lines, and which notes you can use to input them. This may help a bit, and particularly if you have chosen instruments where the individual sounds haven't been allocated yet! The sound 'unpitched.exotic.silence' is often found in percussion staves, and although not very exotic, it is silent!.
For existing pieces you might have to import one of the Standard v.5 House styles — at least the Instrument Definitions part.
One difference from version 4: sounds not allocated in the Staff Type dialog won't play back anything. In v.4 they played back whatever the MIDI sound was for that particular instrument on that particular pitch.
I hope this is of some help. I'm still struggling with unpitched percussion myself (though I can usually get it to work in new pieces, updating existing scores is an appalling amount of work).