Until your burm is better established at feeding I would continue offering it what it will eat. Once it's feeding regularly you can convert it to rats - or even rabbits if you can get kits or fuzzies - by chain feeding as you did before. Eventually the snake will associate the rat/rabbit smell with food but it can take a few chain feedings for it to make that connection.
I converted four snakes from rats to rabbits by essentially chain-feeding a small (4 oz or 120 gram) rabbit behind a small rat, the longest holdout took six months but she was an adult BCI.
Otherwise make sure your snake has appropriate heat and humidity and handle it as little as possible until it's eating regularly; baby burms get stressed as easily as ball pythons do.
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