Get info is greyed out in iTunes

This appears to be an issue that many people have raised over the years without any common solution appearing to work. When I right click on a song in a playlist, and select Get Info, the information in the tabs is greyed out and cannot be changed. So, I cannot add lyrics, or fine tune the playback volume.

I've checked the iTunes folder in Finder and this shows I have Read & Write privileges (I'm the sole user and administrator). I've even searched for individual songs in finder and checked that I have read & write privileges there too (I have). Any ideas, please?

iTunes 12.1.2.27

Yosemite 10.10.3