Hello- new to the group as I just bought my first M4 and brought it home last night!  Gun is brand new.  Got it home, followed the directions to clean, lube, and assemble.  Followed some of the direction here and applied a lot of lube.  Right away in working the bolt, something was not right- it was hard to pull back and kept sticking back.  Thinking I had done something wrong, I disassembled, watched a bunch more YouTube videos on disassembly/assembly, and then very carefully reassembled.  Same