Why does transfer to iPhone fail because some of my "items cannot be played on this iPhone"?

Hello. I have a number of audio files in .mp3 format, as well as some .mp4 videos that I have succesffuly imported into iTunes, and can play/watch these files in iTunes with no issues. However, when I attempt to sync to my iPhone 6, the transfer fails because the items "cannot be played on this iPhone." I have tried sorting them as podcasts, audiobooks, etc. but the transfer continues to fail for both sets of files. Any help would be greatly appreciated. I have also tried Skysoft TunesOver but the transfers fail there too. Finally, I tried converting the files to AAC but that did not work either.