Musescore2.1 won't let me change percussion noteheads

Hi. Musescore2.1 won't let me change percussion noteheads. I've tried using both the Inspector and the palette, but in both cases trying to change a normal notehead to a cross produces no result. The strange thing is that, in the Inspector, changing to small notehead works. So why not repsond to changing to a cross? I'm in the process of creating a simple chart introducing the Cha-Cha rhythm. The instrument in question is the cowbell, on its own one-line stave. However, this problem appears to affect all other intstruments too.

Appreciating any help on the matter!

James