Hi Bad dog,
Welcome to the wacky world of M2 problems. Looking at the video it appears that the cocking handle is not engaged correctly in its slot in the lower receiver. If you have separated the upper and lower receivers recently you may not have engaged the cocking handle in its slot in the lower receiver. You have to engage the brass roller( on the blowback unit) in to the parallel slot in the cocking arm then as you lower the upper receiver/barrel assembly onto the lower receiver you must also make sure that the rib on the underside of the cocking arm is engaged in its slot in the lower receiver. If it is not this will cause the problem you are experiencing. Remove the stock/remove the 3 screws holding the upper receiver/barrel assy. / gently lift the upper a couple of millimeters or so then see if you can push the cocking arm inward towards the receiver, then when it is in its slot lower the upper receiver ond re-fit the screws. I hope this makes sense to you, if not p.m. me and I will give you my phone number.
flatrajectory.
"REMEMBER, LIFE IS NOT A REHEARSAL"
O.K, If your happy that the cocking arm is in it`s groove in the lower receiver then the only way the arm can come away from the brass roller is if the blowback unit ,( what would be the bolt on a real steel ), is if the blowback unit is somehow rotating out of engagement. I cannot think how this could happen. You say there is stiffness in the bolt when moving forward. Another commom problem with this gun is the Roller in the nose of the Hammer can come loose, as the pin that holds it in the hammer can come loose and jam up the action.
Obviously this means separating the upper and lower receiver again. Whilst you have it apart to check the roller in the nose of the hammer try cocking the hammer with your thumb then see how the blowback unit slides back and forth in the action. I grease the lower part of the action a little so that the blowback unit slides freely.
Also you can increase the return spring tension for the cocking arm. The cocking arm is returned by a very long spring on a long thin rod next to the cocking arm .This rod has a circlip and three grooves. Try moving the circlip onto the middle groove, this will give more tension to the return spring.
flatrajectory
"REMEMBER, LIFE IS NOT A REHEARSAL"