I would remove the buffer tube from the lower, then just drop the buffer in the tube and see how it moves in the tube. If that works good try putting the spring in with the buffer and see how that works.
New buffer tube, used buffer and spring - buffer gets stuck with spring compressed.
New buffer tube, buffer and spring - same results
How far back is the buffer when it gets stuck?
Thanks! The first AR I built I struggled with that step so bad. I used the plastic bag method and caught that little detent so many times that I almost gave up on completing that build. Here is my first AR, I used a photo of it in the video intro.