Grohedal flushing?

My grohedal cistern does not flush the correct amount of water when the romote button on the push plate, part No. 37 769/37 381, is pushed. Only a small amount of water flushes/is released when either the short or long flush buttons are pressed and they have to be held in manually to achieve this small flush. When I remove the flushing plate and manually lift the top of part No. 42 774 it flushes OK. I know the push plate is OK as I have a new spare one and this gives the same result. If I replace part No.42 774 will this solve the problem. If so how do I fit this part. If this is not the cause of the problem can you advise me of the fault and how to correct it.