Adding additional music to itunes library

For now, I'll assume that you mean "yes, specifically songs converted from YouTube" (and that you haven't tried adding anything else)

Something has changed, but we don't know what.

Either, iTunes has changed and is no longer letting you add YouTube conversions to your library, or the convertor programme has changed, and is now creating a file that iTunes cannot use.

In either situation, copying music from YouTube is an infringement of copyright law (in other words, it's illegal), so if that's what you're doing, then perhaps you should consider buying the songs instead, and thereby helping the musician put food on his or her table.

Are you referring to music that you've converted from YouTube, using a convertor programme, or music from the iTunes Store, CDs, or online stores such as Amazon?