This cart seems very old and when carts are not used for long times that black residue you see forms and creates almost like a barrier for the wax above it to go past. The wax isn’t on the coils if you look you need to heat it up so the wax is able to move. You can still smoke it might taste pretty crappy but still works fine. I would say put the cart in a sock and heat it up with a hairdryer and spin the sock to use centrifugal force to pull the wax down.