What about APE tags? You can see their presence either in a column fairly far to the right in the file list if you have not changed the default installation a lot.
Or open the extended tags dialogue (again) and have a look at the window title - if it says "APE" there somewhere then you have them.
APE tags can be a problem as MP3tag shows their contents in favour of ID3V2 and ID3V1. And if APE tags are emtpy, then you do not see the ID3Vx contents.
Go to Tools>Options>Tags>Mpeg
Untick APE in the sections for "Reading" and "Writing".
Press F5 and reload the files.
To see data in the tag panel, you have to select at least one file in the files list.
If all this does not help, check if you have a definition for the same tag field more than once in the tag panel.
Please note that ID3V2.4 tags are not compatible with the Windows explorer.