10 days is a bit too long for an adjustment period. Have you ever seen the fish pass feces? Is there any way you could show us a picture of the fish? Based on the abdomen and body shape we may be able to tell if the fish is starving or has internal parasites. Fish with internal parasites often lose their appetites and some develop a distended abdomen.
Can you tell us a little more about your maintenance routine? (water changes, water additives, etc)
It may help if you pre-soak the pellets in a little tank water before feeding. You should also try out some different foods. Bettas, like other animals, need variety in their diet and cannot get complete nutrition from a single pellet. Most keepers, myself included, alternate between different pellet brands ( I use Omega One Betta Buffet Pellets and New Life Spectrum) and supplement those with frozen foods, like brine shrimp, blood worms, and daphnia. You can find frozen food in the freezer at the fish section of your pet store. They are quite easy to use, I simply cut each cube into small pieces as needed and only thaw as much as I need for one feeding at a time. One package of cubes will last you quite awhile if you are conservative.
Freeze-dried foods are tempting to buy because of the price and "ease of use"--but they do not give your betta as much nutrition as frozen food due to the freeze-drying process and they can cause serious constipation, especially if not used properly, so you should avoid those.