ZITAT(MacSmasher @ May 19 2012, 04:21)
...Do you need to mass convert all tags from ID1 to ID3?
All playing in the PC with Winamp and Zune software.
THEY are all mp3 by the way.
Isn't this an off-topic as it concerns iTunes?
No, you don't have to convert the tags as iTunes usually does not have problems to read these tag types.
How do the files look like in MP3tag? Do they show the information there?
In what way do the bad files differ from the good ones?
Are they stored on a different device?
Have you (accidentally) applied a filter in iTunes (top right of the window)?
Does a check with foobar2000 or mp3val show any maladies?
There are so many possibilities ... you have to help a little.