When I perform the function boolean the head disappears. What could be the problem? 
Hmm, that generally only happens if there is an issue with the geometry and it doesn't know how to handle it. Try double checking your normals and merging by distance on both objects. You could also try moving the bool around or switching the method over to Fast and see if that helps diagnose the issue.
I switched the method to a fast one and it works, but is it worth leaving it like this and continuing to work?
mmentalzhenya yes you can continue working like that way. It's called non-destructive booleaning.
Further explanation: