Yes it could be the coupler. It could be a bad seal on the bottom, inner O-ring causing gas to push pass and go directly to the beer line. I would try the following in this order:
1) attach and reattach coupler to try to get a better seal.
2) disassemble and clean well, paying attention to...
Several members were interested in a stainless steel version of these twisty things (known in industry as in-line static mixers). Found a couple of places that sell them, but if you were complaining about a hypothetical $14 a piece from that other guy, don't bother clicking on links...