back door not locking

The back door in my 300tdi 90 hasn't locked for a while so I bought a new back door lock assembly and new barrel. Fitted it fine, locked fine outside the vehicle, but when it's in it won't lock. The key turns about a quarter of a turn each way but that's all. I'm baffled as to how it can not be locking.

Anyone got any ideas?

The latch plate needs adjusting, or you need to close it harder, mine does the same if the back door isnt closed with enough gusto

Ok thanks I'll have a look at the latch plate, is that the bit that the door catch snaps around