Apple's habit of rejecting a feed without giving a reason is extremely unhelpful. The most likely explanation for your rejection is that your podcast consists of 'mixes' of (presumably) comercially issued recordings. This is a breach of copyright, and forbidden by the Terms of Use for podcasting. (It can also get you sued if a copyright owner notices and feels litigous.) This sort of podcast, once the Store staff detect what it is, is always going to be rejected.
You can use 'podcast-safe' music (Google on the phrase); if you do this you should make it clear in the description that that is what you are doing.