I had the problem a few months ago, the guys here were very helpful. My problem came from the front drivers door. I was instructed to spray it down serveral times with penetrating oil. After 5 different times of spraying the latch with oil, it finally fixed the problem. After that I sprayed ther rest of my doors and haven't had a problem since. Try that out, it may take a couple of days but it should work.
-Jay-