lightbulb vapes suck, but if you have your mind set on using it then here are a couple tips.
1: make sure to cut the weed up because, yes, only the thc near the surface will evaporate.
2: you should be able to see some vapor come off of the weed when its been heated for a little while.
3: shake the weed around every minute or so to get all of the surface to touch the glass.