The sliding door doesn't lock with the key or the button inside (its the one with the lock separate from the handle). Not been an issue, but last night some scallys opened it up and rummaged through all my gear before stealing the contents of the first aid kit. So tonight I need make sure the door locks, haven't investigated it yet but is there a likely bit to have a look at first?
Probably still worth taking it apart, [with a key in the lock barrel to keep all the bits in unless you like collecting little springs from the ground.. ] , and clean off all the crud and then replace with mininal lubricant e.g. proper graphite lock lubricant as this collects dirt slower.
I had this happen to me. Lubed it up but you should also check that the latch is actually engaging the striker at the front of the vehicle properly. Mine was not catching over the lip enough so I could not lock it. Sometimes it would go over (if I slammed it hard enough) sometimes not. I adjusted the striker a little so that it always went over - locks every time.