Problems with Downloading purchased content to IPad.onths.

I have an Ipad, third generation with 64 GB and cellular capability. I purchased about thirty items, songs and movies for it. For the last four months about fifteen of the songs I purchased do not download to the IIPad. I paid for these songs and feel I should get to listen to them! Other material does download successfully, but not these songs. What can I do to get the material I purchased?