New Geberit wall hung runs occasionally after flush?? - worse with faucet running?

We have a new geberit concealed tank and duravit wall hung toilet.
Occasionally, after flushing, the tank continues to pour water into the bowl so that the tank does not fill.

I assume the flapper has not closed?

The problem with these tanks is getting access to the parts - we can't even see the flapper.

Anybody seen this before - any ideas?

It seems to occur more frequently if the shower is on, or a faucet is running - but its also occurs when neither of these are on.

The original plumber who did the install (as part of remodel) didn't have a clue and is not being helpful
cheers
Edward

 

Anybody out there....? My toilet still underperforms

Bumping the thread to get help.

Will also contact Geberit customer support to see what they can do