I had this happen on both passenger doors, and it got to the point that the rear door wouldn't open at all.Apparently it's a fairly common mk5 fault, the locking mechanism just gives up.IIRC it was not cheap to sort, a few hundered per door.
Could be done cheaper if you're happy to break out the tools?https://www.ebay.co.uk/itm/REAR-LEFT-Door-Lock-Mechanism-compatible-VW-GOLF-5-MK5-JETTA-TOURAN-CADDY-MK3-/121156844322That's an example link for a locking mechanism so check the part numbers before you buy.Here's the DIY I used.http://www.mk5golfgti.co.uk/forum/index.php?topic=34343.0