Sounds like a relay in your central locking control module has packed up, i'd suggest running a new wire to another solenoid which should sort it - provided it doesn't draw too much current...
Listen to the door that doesn't lock whilst trying to lock it. Does it make a noise? If so the wiring is probably fine and it might just need adjusting/lubricating/un-jamming
had this on my double cab....strip the door, spray brake cleaner fluid into the mechanism while moving it manually loads and loads, then check and adjust the rod length if necessary, mine was way out!!! this worked on my one rear door and touch wood still is, but the other side needed a new actuator at a cost of nearly fifty squids....still cleaned that mechanism as well, now all seems ok on mine and has done for some time....hope this helps