How to properly slow cook lamb's head?
I don't have any experience with cooking heads, but the problem sounds familiar enough.

The quality of meat in restaurants is often far better than what you can buy at a regular consumer market. What you - a regular layperson - buy as a "lambs head" might actually be an old rams head. What a restaurant chef buys is fresh meat from suppliers the chef trusts to deliver high quality. I don't want to imply that you aren't able to distinguish the head of a young lamb from that of an old sheep. People just try a lot of things to earn more money for less quality wares. 

The reason why I suggest your sheep might have been old is that the segments of the skull are seperated at birth and fuse with age. From a certain age (I'm not sure what that age is in sheep) the upper skull is one solid bone and no amount of cooking can seperate the individual segments. Especially male sheep have a very solid skull to protect the brain from damage while ramming their horns into opponents.