The reason why you keep finding a Medicham is because in white 2, there is 15% chance of finding a Medicham over any other Pokémon in the Hidden Grotto of Pinwheel forest. There is only 5% chance of finding a Bagon so it is very normal that you keep finding a Medicham. My advice to you will be to keep trying everyday and to have a lot of patience. it took me 30 days to get a wild Bagon in my Black 2, but it was worth the time.
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Hope I helped and good luck