You may need to explain further what it is you mean to ask.
If you mean Voodoo doll in its religious form within the practice of Voodoo. I can not help you. I know very little about Voodoo and its practice. What I do know was seen on a documentary of the religion.
If you mean poppet or sympathetic magic using a doll, then I am more able to help, but I still have trouble with your question. Which is how such a doll can lose an enchantment? Do you mean to ask how such an item can be rendered ineffective?
Answering these questions can help us better understand what you mean to ask.
If not fed the energy it needs any magical casting is at risk for dissolving. A doll can be used to counter act a different doll and acleansing, banishing, or binding can also render a doll useless. If you create a doll please remember to put a protection on the working, whether it's for healing or to curse protecting your doll means protecting that which you desire.