How can I fix my toilet flush?

From your description I gather that your cistern has the siphon type of mechanism that has a semi-rigid membrane over a plastic grid inside the siphon which lifts the water past the top of the siphon and the rest of the water keeps flowing through the siphon past the lifted semi-rigid membrane, to complete the flush. With use this membrane might have been torn and is not doing its job. Be bold, remove the siphon and put a new membrane. Mine worked for a long time now. If the siphon cannot be dismantled without damage you might as well purchase a new siphon. Hope this works. Good luck.

DIYer A.V.
July 2008