Chunk of emails cannot be found

Hello all,

I have searched the support community before posting and although I have found many similar cases I cannot seem to find an answer to my question.

I am searching for emails that were archived a few years ago but they are not showing up when I perform the search.
I even scrolled through my archive and to my surprise I have got a whole bunch of emails missing from 2010 through 2012. Prior and after that is all good.

Can anybody guide me as to where these emails might be? Or how to find them?

And before you ask, I did not delete them, they are too important.

Many thanks in advance for any help you can provide.

Cheers,
josh