I have downloaded several books to my iphone 4s from the apple store, and have synched them between the phone and the computer. They work fine for a while, but after a while, they all revert to "pausing", as if they were not synched, and this is driving me nuts as I can't read the books when the phone is not connected. I have to constantly re-synch in order to have access to the books. How do I correct this?