Are you sure that you have the correct length buffer and correct length spring for the receiver extension in play?
Forget all about upper receiver, The void depth of the receiver extension dictates what length buffer and spring are to be used in that lower.
Telescoping butt stock uses a carbine buffer, and carbine spring (10.5'), while a fixed stock will use a standard full length buffer and 11.75" spring instead.
As for H buffer in a telescoping stock lower, only if you are shooting 69gr plus ammo, and if just 55/62gr ammo, then should be running the 2.90z buffer.
Short of this, they maybe hammer wedging or FCG springs not installed correctly, and if this is happening, the back top tail of the trigger/disco is going to have nasty impact marks from the hammer tail.