If the pneumatic push button stops working it is probably because you have an air leak somewhere. This could be in the push button bellows, the air tube seal or the clear flush valve bellows. It is better to know which part is leaking before ordering the spare part.
If you can get access to the top of the cistern lift up the 'cradle unit' which holds the clear plastic bellows (it slides upwards on grooves in the cistern) and extract the cradle, bellows, valve, tubing and push button bellows as one unit. Hold it underwater in a bucket, press the push button bellows and discover where it is leaking.
Then order the correct part (Google 'grohe push button') and refit.
Jamie
May 2020