My push button toilet won't flush - it's lost it's "kick"?

My push button toilet is struggling to flush. Whereas previously one push would create enough power to set the flush mechanism going,now it's like the "kick" has gone. Repeated pushes in quick sucession sometimes kick start the flush, but this is failing now.
It's an en-suit toilet which has the cistern built into a unit making access very difficult. I've tried having a look but I don't have a clue what I am looking at! We can't get a plumber around for ages as they're all so busy - any help out there?!