Are you using a flash drive? If so, I would format it to fat32 it it's not already and compare the file types (MP3, WMA, etc.) to see if they're all the same. I haven't searched for compatible/incompatible music formats for the HRV, but I try to stick to MP3 files. I have easily over 500 songs on mine in many folders and so far I haven't found any that the HU cannot play.