Now thats strange. I ve played 5-6 characters and i never had a problem with that quest. I cant burn the witches head and i am stuck. What should i do?

There be bugs with the http://www.uesp.net/wiki/Skyrim:Purity quest. Since it's not straightforward and I'm not entirely sure where you are in the quest and what's been completed I've provided the link. Hopefully that will give a spark of inspiration. Good luck!

I've burned the first head and finished the companions main quest. Now i have four heads but it seems that i cant activate the flame. One other thing that i've noticed is that the heads are sometimes marked as essential items, sometimes are not. When i enter a new cell they become non essential. Then if i drop them and pick them up they become essential again. This is strange