If the reel won't rotate smoothly the most likely explanation is that the bedknife (fixed blade) is touching the reel in some places and not in others. This is very common. To check whether this is the problem you need to isolate the reel's drive from the engine and the drum (roller) drive. You can do this by removing the reel chain, or perhaps by declutching both the engine and the drum. If one or more, but not all, of the reel's blades are touching the bedknife you will almost certainly have to have the reel professionally sharpened, but don't do this until you check the reel's bearings. If it has slack bearings you will not be getting a proper assessment of whether it is running slightly eccentric.
The cause of reel eccentricity is not all that likely to be a bent reel axle, though that is possible. It is more likely to be slack bearings, or eccentric blade wear caused by running the reel with slack bearings in the past.
Your mower cannot work properly until it has good reel bearings, a sharp, concentric reel, and a flat, sharp bedknife. However you need to sort these things out in the right order or you may pay for a sharpening job then ruin it immediately when you first run the mower.
