Those must be some weak-springed blinkies. I've seen this problem (batteries losing contact) in hand-held flashlights with D-cell batteries, but not in blinkies. I always have messed with the battery contacts and shimmed connections first, because that's the easiest and the first thing I think of, but it's never been the answer except on those big old flashlights. Almost all of my blinkies are mounted sideways though, so perhaps I wouldn't see that particular problem.

Every time I've seen this issue in headlights, ultrafires and little flashlights the fix was cleaning the threads holding the switch mechanism part. A couple of times on cheap AAA flashlights the switch was bad. Mot blinkies - I think - have the switch built in to the circuit or at least the body so it wouldn't be that problem, although if it's directly mounted on a circuit board the first thing I'd check is the solder connection. I'm just saying that it's not all that cut and dried that it's from the batteries moving around.

Perhaps the quick and easy try for a fix would be to mount the blinkies sideways.