What are the file sizes of the mp3's that will not work?

Try a back up player to see if they play or not ... only a basic simple player interface but can play many more audio formats than iTunes is Toolplayer, find a link to it here: Alternate Audio Players & support plug-ins

There are a lot of either fake or deliberately corrupt files being spammed onto the network. Catch a look at these: (1) Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see) & also (2) mp3 Bit-Rates Versus File Sizes (click on link)

iTunes cannot play mp3Pro format, though I suspect that format is gradually becoming rarer these days, with any luck at least.