I have a concealed cistern with push button air flush?

When you push the button it fails to flush ( its gradually got worse over the past weeks) the problem is quite simple in as much when you press the exterior button the air pushes down a little concertina type bellows which expands to push down on the main valve which then flushes It moves but not enough to push down on the main valve
My Question is could there be a problem at the exterior push button as well as or instead of the valve end
Ive been told that the cistern may be a Grohe
Thank you Alan Henson