I have a PFS cap over some worm castings for dirt and I've noticed that it's gotten crusty. Not just on the very top, but throughout. I try to stir and break it up when I plant in it, but it goes back to crusty pretty soon. I do have trumpet snails, but they don't seem to make that much of a difference. I've noticed that sometimes plants don't root well when I put them in, but other times they're fine. Is this normal? Is it harmful?