Curious on if you have new door gaskets. Some versions (aftermarket) don't fit very well at first. According to some they will wear in, and fit better after time. Others have broken door parts trying to slam them shut. Some claim the only cure is a proper fitting gasket. All agree OEM is best, but it is very expensive.
I agree with Patrick. If you have been spraying lithium in the mechanism for some time, you need to pull it out and clean it. Don't forget to clean both parts of the door catch (door and chassis). Also, make sure that you aren't dealing with a door adjustment problem. Watch the door as you close it. Does it rise or fall as it contacts the catch? Does it sit flush with the rear quarter panel? These are clues.
Sounds like your Door Catch mechanism is set just a tad in too deep and high.
The Latch mechanism is probably striking it and trying to rotate to grab, but since it is high the door jumps a bit and in the ensuing rebound the latch then just doesn't get rotated far enough for the cam to swing and lock.
Check your door alignment by comparing the door edge to the rear fender edge. If as look on the body from back to front you can distinctly see the door panel edge, you are definitely out to far, if you can feel it, you're just a tad out. Adjust at the catch.