iTunes gift card can't be found on the Store

I brought a iTunes gift card £50 and the card was activated,

when I went to add the card to iTunes Store it said it wasn’t found,

it turns out the card was only active for £25 “when the card is a £50 card “

I have gone back to Tesco and they say the are a 3rd party and cannot get my money back and they cannot figure out how the £50 let them only put £25 on the card,

and iTunes cannot help as they think I’m making it up,

but the PBH number on card and the receipt match up,

and because I only pay £25 and the card should only be a £25,

so £25 is lost

[Edited by Moderator]