Thanks, Macaw, for that info.
It's been happening a lot more lately, and I phoned the dealership today; I'll have to take the car in next week in order to fix this problem -- the WD-40 isn't always doing the trick, tho I think it's helped a few times. I actually had to drive the car on the highway today with a bungee cord holding the door shut for awhile. Eventually the latched worked (phew), but it was a bit scary (and noisy) till then. I think the problem may have something to do with expansion in heat.