Have you been using abrasive material to clean your
toilet? The glaze on the porcelain could be scratched.
If you have a pee flush the toilet,urine causes scale.
The cheapest way to clean your toilet,go to Lidl or Netto and get a cheap bottle of vinegar,get as much water out of the toilet bowl so it does not dilute the vinegar,pour in vinegar and leave it in over night, in the morning add a tablespoon of bicarb of soda,this will froth up and it cleans under the rim,leave for an hour,then Bobs yer Uncle the jobs done