wirquin jollyfill full flush button stays down?

I have a problem with my Wirquin Jollyfill Toilet cistern.

When I flush with button 1 it flushes and refills ok

When I flush with button 2 (full flush) it flushes, but as it refills some of the water comes through the pan because button 2 has not come back up on its own so have to press button 2 again to get it to come back up again, but it wont do this on its own.
I have replaced the dual button assembly but this hasnt fixed it.
Can you please advise on how I could remedy this?