Prius stereo will not play ALL the songs on my Flashdrive

In preparation for moving songs to my flashdrive from iTunes I converted them all to mp3 format as some were AAC and Apple lossless. I moved about 1000 songs from iTunes to my USB in batches of about 250. I set up one giant playlist alphabetic by song title. When I play the songs on my car stereo, only the first 250 will play. When I try to find the other songs, the car stereo seems not to find them. The only file that appears on the car's media screen is the Root Directory or Root file, no other files will pop up. When I go into the Root Directory, the 250 songs are indeed listed there, but none of the others. Is the car stereo recognizing each song as a file? …it seems that the first 250 songs are individual files within the Root Directory (?) I can see ALL the songs on the flashdrive when I access it back on my Mac. Or should I have created individual files or playlists BEFORE transferring to the flashdrive? I have searched this forum and google with little success. Any help is greatly appreciated! Thanks.