The problem is that some maps in OSU have only one difficulty, and they need two maps with different difficulty to work correctly. I just when I made my bitmap map, it didn't work for me until I realized that there is both an easy mode and a difficult mode.
Therefore, if you download a map from this site, you need to make sure that there are two maps in the bitmap map, where the first map has an easy difficulty, and the other one has a difficult one.