I had the same problem
actually, i still might. My daughter's betta was only eating 1 pellet per day. He used to eat 4 or 5. I got him a larger tank 2.5 gal, fasted him for a day, which i understand is ok and even sometimes beneficial to do a few times a month. Even after fasting, 1 pellet, and he spit half of that out.
I have read they are picky eaters, so I went and got him some freeze dried brine shrimp. That seems to help. have not yet tried the freezed dried bloodworms. The plan is to mix up his diet, which i also understand is important.
I am new to fish, so don't take my word, but i have done a little reading, lately.
The pet store told me the freeze dried blood worms work very well, they come in cubes, keep them in the freezer, you can feed them a quarter cube after u have allowed the cube to thaw in water. It was a bit too messy and too much work for me, actually my wife would have been grossed out keepin them in the freezer, so i declined this option.
I'd be curious to know if this helps.