As mentioned before, it is probably most likely a glitch, sometimes it can happen if you have a small character, or if you have a small character and you're falling through a rock on someone else's screen view~
Though if that isn't the problem, then it's most likely that you have a few object files missing, collisions for example? They are what attach to the object to stop you falling through them :3 Though, has this been happening a lot lately or just recently?
What exactly do you mean "falling through" rocks? It is you can't get on top of them? Or is it that maybe you simply can't stay on?
I'm really no expert on this but I do know a thing or two that might help. You may want to make sure that your character isn't too small. If the character you're using is incredibly small in size, things glitch easily and can cause it to look as though your character is on the rock. If not, I might be that you simply need to practice jumping up. My friend had this problem when she first joined. She jumped too far or too high and slid off the edge of the rocks. If that's the case, then I just advise to practice practice practice. Eventually you can learn how to do it a bit better. Try not holding down the front arrow too much, this might cause you to go too far forward.
Hope that helped a little! I'm sure others have many other pieces of advice better than mine. Since, like I said, I'm not an expert.