Pep manager does not know how to handle her job

Purchased a device at PEP north west and device started giving me issues after the 7day policy of theirs, i went to the pep in my area took the phone with the box all accessories etc and the receipt and it's still within the purchase date and they are telling me they can't exchange or refund knowing very well it's an issue with the device itself I'm even willing to pay the difference for a different device.