poor flush from Grohr Cosmopolitan push plate?

I want to scream. I have had 2 plumbers. One part fitted the bathroom then was unable to complete the job. The second was brought in by the tiler. He was asked to check the work of the first plumber and complete the job. I came back home to find concealed cistern tiled in but when the flush was pushed a pathetic amount of water fell into the bowl which would not flush much away. The plumber has no idea what to do. The suppliers refuse to help ( happy to take £3000 for a bathroom suite)
Can anyone help me? How can I adjust the flush from inside the little box? Plumber blames supplier, supplier blames plumber for not checking before tiling.