Does your filament get stuck inside the j-head when feeding it? If yes, then that is the problem I had with my chinese J-head. There can be a couple of issues causing it but the bottom line is the same - things do not line up.
I took my J-head apart and found out that the center hole in the thermal break was not in the center, it was off to one side. So I ordered a replacement heat break (and couple extras since they are still chinese)
The second issue is gaps between the nozzle - heat shield - heat sink
First thing to do is take everything apart and clean the threads inside the heat sink and heater block with an M6 tap. Ideally get a tap with a flat end so you can make sure the thread runs all the way inside the heat sink - mine had about 6mm gap between the end of the thread and the bottom of the hole inside the sink.
Screw your heat break all the way into the heat sink.
Next screw your nozzle inside your heater block. Then unscrew it about 1/4 or 1/2 turns. Now screw your heat break into the heater block until it meets the nozzle. This is important part - until it meets the nozzle. Now tighten the nozzle back up and tighten the small hex screws (if your hot end has them) to tighten everything.
Test with a piece of filament to make sure it slides in and out without problems. If you meet any resistance, take a matching drill bit and try to smooth things out. Keep in mind - after you use a drill, you need to take everything apart again to make sure you do not have any debris inside your hot end.
