So i found the main difference between TM/KJW and WE M9 mags was that the gas router on the WE are flat whereas the other two are curved so it isn't able to form a proper seal with the nozzle. Replacing the router with a TM spec one is also quite finicky so I have yet to get the WE to work in my kjw.
Something to bear in mind if your double bell is 1:1 with TM.