polo 98 central locking

hello guys and gals.

This is my sisters car that i'm looking at for her, and at first the central locking didn't work at all. After reading on here it seems the main locking units aren't very clever and i replaced hers with a scrapper part.

Woohoo central locking works. Or nearly i should say.

The nearside rear door is very slow to lock. I wd40'd the locking bits to free em up a little and it is easier to move by hand now but still, it is slow when locking. It unlocks fine! But we have to turn her door key twice into the lock position in order to lock the rear door. Any idea's why this could be?

Also her old locking unit does try to do what it does but it's just weak is it repairable?