Bonuses may be declined if certain stipulations are not met and your purchase, therefore, doesn't qualify for the bonus.
There are a few reasons that this may have happened:
Your purchase was not made within the bonus timeframe
Your purchase has not met the specified minimum spend
The cashback that the bonus has linked with has been declined
The cashback claim that the bonus has linked with has been declined
Sometimes a cashback claim may take some time to resolve; this may mean that the bonus expires before we are able to link the cashback/claim as it has taken so long to confirm the claim.