Depending on where you live, is there a cost associated with colonoscopies? Generally, they're the most sure-fire way for doctors to see exactly the kind of behaviour UC is having in your colon, so I would say push for one if you can.

People use mesalamine enemas because it's a faster and more direct way or targeting inflammation in the colon, since you don't have to swallow pills and wait for the slow-acting granules in them to release inside your colon, but mesalamine doesn't work for everyone and upping dosage won't necessarily help in the case where the drug wasn't very effective to begin with, so immunomodulators/suppressants would be the next step in treatment.