Suggestion: Could you make the Line of Sight check based on the viewing height of the camera rather than the character's head.
Not many people would be in FPV and mobs get hidden in small hollows which are not apparent from a normal camera position.
This is particularly evident when fighting the asps in AVP or rats and other smaller mobs.
It feels buggy when trying to cast and nothing happens, when you can clearly see the mob, despite probably being "accurate" if you were in FPV.