Originally Posted by FBinNY

Often slightly mis-aligned threads on the BB shell make the second side very difficult to tighten. I always take a minute to check the threads on both sides (both the BB and the BB shell) by trying each side separately. If each side threads by itself, but not together, then you're looking at a thread alignment problem.

Sometimes, besides using grease, it can be easier to thread both sides together at the same time, then when nearly all the way, stop and finish the fixed side to the bottom, then finish the 2nd side.