do not use alot of oil in the gas system during reassembly. Then clean up the piston and remove the transfer rod and spring and clean it all up.careful with it, it is spring loaded. If you have an Ak or SKS cleaning kit, cleaning the gas port with the included tool would be advisable as well as cleaning out the gas block. You can keep swapping back and forth with a brush and a patch till it comes clean. 22 brush for the small end if it's gunked up. 45 bore brush and CLP or Hoppes9 works good for the piston tube, you can also use a. Cleaning the gunk out of both the tube and gas port. It shouldn’t take a lot, so no need to wail away on it.you just trying to seat everything forward, back in it's original place. You can.'carefully' try to tap the transfer rod forward(the rod that sticks through the receiver) with a hammer and appropriate sized punch to see if you can drive both the transfer rod and piston forward a small amount. The lever that unlocks the gas tube has to be in the perfect position to let the end of the gas tube past. Since the release cam 'is' turning, the one thing really left is the piston sticking slightly out of the gas tube. Either one of these could cause the tube to hang up on removing it, it doesn't take much of the piston sticking out to hang it up. Even old dried gas and powder residue hardend shipping/storage grease could be in the tube and never was cleaned. If surplus corrosive ammo was used or it's digested alot of dirty Russian ammo, there could be either or both, rust and/or carbon build up in the gas tube not allowing the piston to go fully forward in it's normal resting position. If it is a recent purchase, the previous owner may have never removed the tube and cleaned it correctly. My questions would be, has the tube ever been off since in your possession? Is it a recent purchase you are trying to clean? Has someone modded, or stuck an aftermarket tube on it.sometimes they need fitting to fit loose.

If the spring is weak or broken, it may not push the end of the piston far enough forward to clear the end of the tube.

The transfer rod is spring loaded and should push the piston back fully into the gas piston. The transfer rod strikes the face of the bolt body, causing it to cycle. When the rifle cycles, the piston slides back, pushing the transfer rod with it. The piston is (or should be) fully in the gas tube. What you see at the receiver is the transfer rod.

Sks how to take the gas system off