I have painted a few coats of paint on my bathroom ceiling. It's
still a bit patchy in one corner. Is there a good reason why I should
do a whole new coat or is it OK to paint just the corner again?

What paint did you use?

If you use something decent like Dulux Trade or Farrow and Ball, then there
might be a problem with the ceiling. If it was B&Q value, then you could
paint 10 coats and it'll still look rank.

Christian.