Is it possible you used music without the standard payment to the artists? I realize it is quite possible to find lots of good music that does not require royalty payments, but ... Listen to the trailer at the end of any of my recent shows and you will hear me explain my license to use the music I use. They may have spotted a piece they know requires a license and discovered you have not paid.
Just guessing. The usual rejection is due to not being as clean as advertised. But, you say this is not true.
Dave M