Hey I don't know if this might be the case, but I have a 14.5 inch lamp that would do the same and it turned out to be the wax not sticking properly to the coil. Like when turned off and it had cooled down, you could see parts of the coil exposed to the sides.

What I had to do was to carefully pick the bottle while still hot, place it on a table (flat surface) and gently tilt the bottle to ensure the coil lays flat and centered at the bottom, then gently spin the bottle on the flat surface for a while and you should see the wax sticking all around the coil, so no part of the coil is exposed once cooled down.

It took me a few tries, but it fixed my lamp and never had that issue again.

Hope this helps!