When you do an offer for free diamonds that is not credited immediately & then switch accounts, TapJoy and Fyber are not able to register that you are playing on multiple accounts or which account you started the offer from.

When the offer is complete, the rewards are sent to your device & are likely credited to the account you are currently on or last logged into.
Unfortunately, the vendors do not share any information with us regarding your offers so we are unable to verify or transfer them for you.
This can be avoided by not switching accounts until an offer has been completed & credited to your account.