I had identical problem on my 2003 Navigator. Upper and lower latches. I think my sticking was caused by overlubricating. I sprayed a rust remover on the upper and lower door latches One at a time I manually latched the mechanism by pressing it closed with my fingers. Then I released it with the outside door handle. It took about 20-30 times for each latch. Then I sprayed a brake cleaner on each (upper and lower) to dry them out. I now lubricate with graphite. The grease I used to use apparently attracted dirt, thus making the latch stick. So far I have not had a problem.
