Excuse me, if anybody else asked about problem like this earlier (I didn't find any topics) ...
I'm building my own power-up according to Trello API documentation. I made a button (which will get some functionality later). I can see it in a browser, but in mobile apps (neither in Android nor in iOS) it doesn't appear. If I activate another Power-Up (like Custom Fields, for instance) - everithing is ok. Are there some limitations?