Penance - I’d suggest you do something similar to what’s been suggested above.
I’d take the springs, sweeper, the burrs and collar out again, and make sure you put the sweeper the right way up (solid face facing you).
Without putting the burrs in, nor the springs, put the collar back and screw it in, carefully not to cross thread it. Is it moving freely? Does it look right?
If so, remove the collar, and proceed with putting the lower burr in, the locking pin, the sprigs and the top burr carrier. Screw the collar back in, careful not to cross thread as this time the springs will push back.
Does it look better?
If you are still stuck, happy to talk you through it after work. 👍 - let me know and I’ll PM you.