Can you screw the top ring in enough?
when inserting the stem does it offer resistance?
Did your TM work well before or has it always been like this?
(edit: Are you sure you have this oring in place?)
Didn't he by any chance fall to the ground or take a hit?
check the alignment of the top plate...
disassembling this part you should have:
1 ring
2 o-ring
3 aluminum tube
4 heater screen
With this all disassembled, check (by running your finger around) if the oring that seals the top plate and the inner glass tube, are all well aligned with the top plate...