If it fires continuosly it could be due the "Axis's Spring Holder" is making contact with the venting cap, just make sure it is well screwed in the Axis post:
The "Axis's Spring Holder" is the first piece on the following picture, from left to right:
If it is not firing at all, could be due the center pin on the
kayfun is not making contact with the center pin of Just GG.
Unassemble the 510 connector of Just GG (Just GG Top Lid), connect the Kayfun and push the center pin of the Just GG 510 connector until it makes contact with the Kayfun's center pin:
Push this part with the Kayfun connected (it should make contact with the Kayfun's center pin):
Push the Just GG center pin (collector's tank center pin) and screw the Just GG 510 connector / top lid (with the Kayfun connected), you should see that the center pin on collector tank coming out a little, that means that it is making contact with the center pin of the top lid (Just GG 510 connector), maybe you could need to unscrew a little the small adjustable screw of the center pin:
Push this part before connecting the Kayfun (that is already connected to the Just GG 510 connector / top lid):
Adjust this screw if is needed: