Review the amount of storage your phone has and the amount of data you currently have on it. Often you run out of space ;-} Something will need to go to make room.
The other direction here is to forget copying your music and vids. If you have iTunes Match you can leverage Apples servers to host your music. That way you can stream your music & vids over the air and not use your iPhones limited storage. You will use your LTE or WiFi connections to stream your content so you will need to see if your carrier allows free streaming like T-Mobile.
One of the other great things here is if you left the track titles as offered out by Grace Note Apple will substitute their burnt copy of the track as well as break the DRM of any music you had downloaded before.