Dual flush toilet won't refill

We've got an Ideal Standard Space corner toilet with a dual flush system. The flush button broke ? it turns out as a result of the flush plate not being aligned with the button housing. In other words, the plastic screw had originally been forced into position and the bent rods eventually broke. So, we're replacing the dual flush button. We had to straighten up the cistern tank first as it wasn't level and the valve canister was just too out of whack. Now it's sitting much better. We've also adjusted the valve canister height in order to fit the securing plastic screw. BUT the cistern will now not refill. The water is running but it seems to run straight out again. What is causing this? However, without the flush button screwed in place, all the mechanisms seem to be working fine. We are baffled as to why screwing in the flush button is messing up the whole system. Any ideas? Thanks.