If you are certain the “format” is compatible then it could either be the iPhone, iTunes or a hardware issue (eg. cable).
Force Restart the iPhone and make sure it’s up to date, then check iTunes and see if there any software updates. If you have a spare cable try that and plug in to a different port.
You could at one time drag the file from Finder direct to the iDevice in the iTunes Sidebar, in other words the content didn’t need to be in the iTunes Music Library, if it works then that would indicate a iTunes issue and not file format or iPhone problem.
http://osxdaily.com/2013/08/26/copy-music-directly-to-iphone-ipod-without-adding-to-the-computer-itunes-library/
Not sure if this discussion is of use, but might be worth a read just in case you recognise any similarities.
https://apple.stackexchange.com/questions/136375/song-cannot-be-played-on-this-iphone
Hope some of the above helps.