Problem with Enclosed WC cistern possibly pnematic.

[Penelopejean]

Hello all,
Wonder if you can advise on an enclosed WC cistern, all tiled in nicely. It could be the Screwfix type with the big dual flush plate on the front.
We are having trouble with it flushing. At first we thought it was a macerator as we can hear a whirring sound and the water in the pan just keeps circulating. It eventually flushes away but very slowly. Any ideas what the problem could be please?
Thanks

 

[muggles]

Could be a split pneumatic section, you'd better hope there's sufficient access behind the flush plate or the tiles will have to come off