Driver side rear door doesn't lock
 

The rear door on driver side of my 2002 LS 430 does not lock with either the remote key, manually, or from inside. The other 3 doors lock but the alarm does not turn on because that door does not lock. That door's lock feels loose and won't latch when moved from lock/unlock position. This doesn't occur very often, but it is stressful that the car is not securely locked when this happens. Thank you for your recommendations?