Door central locking

[Mickw]

Hi has anyone had problems with the rear passenger central locking not working properly, all other door locks unlock and lock normally, but I have to press unlock button on key phob twice for that door to open and again twice to lock,it's also intermittent on that door,you can see it move slightly on 1st press on opening or closing, I was wondering whether the central locking mechanism could be stiff inside door or just needs replacing, any suggestions appreciated 

[dezufo]

check at bottom of door where the lock pulls it in also at back of door half way up, it might need cleaning/greasing