Why does the push button cistern when operated continue to flush and not close and fill up (old installation)?

Why does the push button cistern when operated continue to flush and not close and fill up (old installation)?

1 - The diaphragm washer could be damaged (replace or reverse the washer)
2 - The diaphragm washer could be dirty (clean the washer)
3 - the Drop valve could be dirty (clean the Drop valve)
4 - It is unlikely the Drop valve could be damaged but not impossible (replace the Drop valve)
5 - There could be some foreign body obstructing any of the mechanisms relating to the Drop valve (remove any foreign bodies)