Content on iPhone deleting itself

Hi everyone, new here, so hopefully I'll be able to get in everything that needs to be said to help solve the issue.

I've got a 64GB iPhone 4S that is completely up-to-date software-wise and has a good 30GB free. All content is added manually from iTunes to the iPhone (the appropriate box is ticked) and I have ticked the box in Preferences to ensure all devices don't sync upon being plugged in.

Now, here's the problem: when adding music to the phone, ripped from CD's or downloaded elsewhere in MP3 or M4A, the music will transfer over to the phone perfectly. However, the next time the phone is plugged in and I attempt to add new content, iTunes tells me that it is updating files and preparing to update, but it just hangs here. The songs are listed as on the phone, but are greyed out. So, when I unplug and refer back to the Music app on the phone, the songs just skip from one to the other, before eventually telling me that there is no content there whatsoever, despite iTunes telling me otherwise.

This is also the case with any video content, all videos are converted in the right format, but I can never get them to transfer over. iTunes tells me it is updating files and preparing to update, but once again iTunes just hangs and the files are there in iTunes but they're greyed out. I can't access or play them, only delete them within iTunes itself.

Am I doing something majorly wrong? This occurs every single time and a hard reset doesn't seem to have solved the issue.

Apologies if this has been covered elsewhere, I can never seem to find a definitive answer or the situation is different from my own.

Cheers!