That's probably because...
...the tags are missing

Remember that iTunes can store tags in it's own database on the PC and doesn't necessarily write them to the actual files.
Are you absolutely sure WMP and WinAmp work?
Try playing an < untagged > file, then bring up the context menu and selecting 'View ID3 info...'. This will tell you if it is just the rockbox Database or the tags themselves that are the problem.
I've also got something right at the back of my mind that says that there is something weird/screwy/generally unsupported about ID3v2.4, but it escapes me at the moment...