I'm only an advanced newbie, but I'd say it means that the roots need more room, or you didn't plant the clones deep enough.
I'd put more soil on top to cover the root, and if it's big enough I'd transplant it into a bigger pot and add a couple of inches more soil on top.
I had a clone that did that myself. It's still in a 16oz cup so I just put more soil on the top until she's big enough to transplant.
Hope that helps some.