Grohedal not flushing properly?

We suspect that our dual flush grohedal toilet is not flushing due to low pressure.

We took the flushing sytem out the wall (with the 2 connecting pipes - green & transparent) and we think the problem is not with the external chrome flusher but instead with some sort of blockage in the pipes - We only managed to flush the toilet properly when we put our hand in the wall and pulled the lever up manually.

Please let us know what you think.

Thanks

Abby

Feb 2008 (London)

P.S if you think we can't fix this ourselves pls could you recommend a plumber in the area