My tags are not shown

I have a ~2121 MP3 collection, each one of the carefully tagged before it is added to my MP3 library folder.

MP3tag is the best tool to do so, but today I decided to check if everything is ok with my tags and opened my library in MP3tag, and something odd is happening:

Many (I woud say 50 out of 2121) MP3 don't show any tags, but in my player (winamp) those same MP3 appear to have tags, for example the one selected in the previous screenshot is Ace Frehley - New York Groove, and look how that MP3 shows on Winamp:

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what could be happening?

I think that you have APE tags in your files and you read them (now) but you probably have not written them in previous times so they are out of sync with the ID3V2 tags.
As MP3tag shows tags so that APE tags have priority over ID3V2 tags, you see the emptiness of APE.
Go to Tools>Options>Tags>Mpeg and switch off "Read APE".
Then re-read the files in MP3tag and see if it is better now.

that is exactly what was happening, did as you said and everything is ok now, thanks for helping!

I'll try to find a way to paste ID3V2 tags to APE

switch off to read APE tags, switch on to write APE tags.
Load the files
Press Ctrl-S to save the tags - this saves the ID3V2 data also in the APE tags.

doing so, I'm nervous somethign could go wrong