Caves, keys and wobbly bits [possible SPOILERS]

I've generally been pretty successful with caves, but there's one type of cave section I've repeatedly had problems with.  It's the type where there are moving rocks and a door at the end with multiple keyholes, and you find keys under the rocks.  Several times I've found all keys but one, but then can't find the last one even after repeated searches.

I can find things like narrow places with more cave beyond them, and areas which can only be reached by moving a blocking rock.

Anyone else have these problems, or have hints as to how to find that last key?

I think this is the same problem as with the items that open chests - ladybugs, bells, horns etc. - sometimes they do not appear all and you need to exit the town and reenter (sometimes several times) in order to find them. I had a similar problem with keys under moveable rocks, and had to go to the cave several times and move every rock again. A hint - the keys are almost always near the door. Hope this helps.

Thanks.  There was one cave in particular that prompted this post, and oddly enough I got an exact copy of the section on another hero shortly after.  The  seond hero got through it without too much difficulty.

When I went back to the first hero I did eventually find the last key.  It was under a wobbly rock and also very well hidden by some stalagmites.  I think I'd seen a glimpse of it before but repeated clicks hadn't got me the key.  Perhaps the active area for key clicks isn't quite the same as that of the visible image?

One tip here, if you're re-checking an area and don't spot a hard-to-see key, on the re-check you may move the rock back on top of the key without realising it.  Then you can search and click as much as you like but you'll never find it!  You have to send your hero round and move all the rocks again.