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https://i.imgur.com/uYewtXp.jpg

The long flush bottom on our toilet regularly sticks meaning water continually runs into the bowl until the bottom is freed up. It’s no great shake to sort out: lifting the cistern lid and pressed the stuck button pops it out but can anyone help me with a permanent fix?

I’ve included a pic of the inside of our cistern. It’s the white button which sticks, the other one moves fine.

Do I need to get some sort of lubrication in there? I can’t imagine WD40 would be any use…?

Thanks in advance