Win7, Some files will change tags, some won't

Hi there!
Wanted to copy some files to my iPod Nano by means of SharePod, but for some files it said: can't copy because of missing MPEG audio header.
So I tried to tag all of the files with MP3Tag, which seemed to work. When I tagged them and saved, reloaded them in MP3Tag, all of the tags were saved.
However, in Win7 (both 32 and 64) only some of the files showed the changed tags. I have no idea why. Some files I can retag, some just won't show the changes in explorer.
Can't find out what the difference between the files.
Tried (almost) everything. Used V3.1, V3.2, APE, but somehow I've got the feeling that I am missing a simple point, that I am overlooking something very easy.
Used to work fine, but with these albums I've got problems.
Anybody any idea?

Adapt the number of columns that Mp3tag shows so that you have a column to display the tag-versions: The value should read %_tag_read%[ (%_tag%)]
When you have a look at that column, check if you have any APE tags.
If so, get rid of them:
Set Extras>Options > MPEG that you read but do not write APE tags. Also enable Delete for APE.
And while you are at it: set the mp3 tags to ID3V2.3, UTF 16, not V2.4.

Then cut and paste the tags of those unwilling files with the functions of the contextmenu of the files list.

Also possible: you have corrupt files - repair them with foobar2000 or mp3val.
Or: you do not have sufficient access rights.

Hi there ohrenkino,
Thanks for the quick reply!
Tag column was visible, I removed the APE tags, but that didn't work. Also I made sure I didn't use ID3V2.4, played with the other two ID3V2.3 options, didn't work either.
I have enough access rights, am owner of the files and logged in with administrator rights. Files are not read-only.
Strange thing: when I removed the APE-tags, 1 (one) file refused.... Couldn't remove the APE tag. Other files were fine (in MP3Tag that is - in explorer still some files had tags and some didn't).

So I tried your last suggestion: mp3val and/of foobar.
Mp3val said that all (?) files had problems: garbage at the end.
Foobar played all files normally, but didn't tag right. So I rebuilt the mp3stram, and now I saw the tags appear in explorer as the process went along. However, when I listened to the files, there were holes, gaps and skips in them. So I think you're right, the files probably were corrupted.
Will try to look for better ones.
Thanks for your help!