Masking problem

Hello, I'm having troubles using masks on LrC. They have been lately improved, which is very good, but they seem to be far behind masks on Ps. When I copy a mask that I previouvly created and modified, then I try to do some more modifications to the copied (or the copy) mask, the program decide to erase all previous modifications, including all the masks. I'm getting crazy! Does anybody experienced the same problem?