Changes to Percussion

I'm having a little trouble with the percussion parts (the ones with multiple instruments). I want to make changes to two particular instruments. However, when I make changes to one instrument and then make changes to the other, the first one goes back to its original form. For example, say I had a splash cymbal note and a Chinese cymbal note. Both start with normal noteheads. Say I wanted to give them cross noteheads. When I make changes to the splash cymbal, it's fine, it has its cross notehead. But then, when I give the Chinese cymbal its changes, the splash cymbal is back to its normal notehead! Then, when I give the splash cymbal its cross notehead, the Chinese cymbal is back to normal notehead!! The point is, it seems that I can only make changes to just one instrument - no more - in the percussion parts. It's extremely annoying. Can someone please have an explanation?