Sankey decarb issues

[a1amberrenae]

Trying to decarb a sankey keg. When I attach the coupler, no gas comes out at all. Trust me, its got a lot of carb, there is beer in there, its foamy when it dispenses. My only guess is it's so pressurized that the coupler has issues letting gas out. The backflow part is removed. I try the same thing on another keg with normal carb levels and gas spews out as expected. Any thoughts?

 

[a1amberrenae]

Update
Letting some beer sit through the coupler into the line allowed gas to exit the gas side. I think anyway.

 

[Bobby_M]

You're pushing the tapping lever down on the coupler right?