I took my first build out to the range today. Shot great but it would not cycle. The Bolt would come back a few inches and stop. The charging handle was very difficult to pull back. At first I thought I need to lube more,but still would not cycle.
I did the Cerakote and thought I may have gotten too much in the upper. After taking the upper off the lower, the BCG was sliding quiet smoothly.
I put it back together and the charging handle was still difficult to pull back.
I did not have my M400 with me so after shooting pistols I went home. I decided to swap uppers from the build and the m400 and the charging handle was smooth as butter. I exchanged the buffer and spring and it felt as smooth as the m400.
So I guess I have a bad buffer and/or spring. Anyone else heard of a similar issue?

I've had to change a too stiff spring before. You can weigh the buffer to at least see if it's the right weight. Springs are cheap so don't be afraid to try a new one.

See how freely the buffer and spring move in the tube. Then try dropping the bolt in by itself. Any binding?
Any wear marks? Bent/dented/out of round tube?

I would be willing to bet your issue is more tolerance stacking with the Cerakote than a bad buffer or spring.

Did you build the lower?  Are you 1,000% certain that the receiver extension is square, straight and true in the lower?