Did not work in 4 ak's - Fostech Echo AK-47 Binary Trigger

Although the Echo AK is a quality built unit I could not get it to function in 4 different stamped receiver AK's. Those AK's are a Norinco AKS, a Lancaster Polish underfolder built on an Armory USA receiver, a Maadi ARM, and a Norinco Mak-90. The issue was the same in all 4 receivers. I believe upward pressure of components in the unit (possibly the hammer) as the bolt/bolt carrier travels over it robs the carrier of the energy to continue forward. The bolt carrier hangs up on something in the Echo and stays stuck to the rear of the receivers. I had to slap the bolt carrier handle forward to get it to go forward, or in several instances I had to field strip all 4 receivers to loosen the bolt carrier from being bound to the rear of the receivers.