I don't feel comfortable telling you to buy ram for the sake of it without testing first but yeah if one of them is bad then yea buy a new set. Could risk buying a single stick of the same specs to save some money but not being in a set is taking a risk if not able to return the ram. Ram sold in kits are tested to work together.
k first off 'unplug power' & depress your case power button
[to get rid of any residual power]. Open your PC case and then touch something earthed [like a radiator] So..
take out both memory sticks & remember which was in which slot
are your memory sticks in slots 1&3 or 2&4 -if 1&3 swop 1to3 and 3to1 -otherwise find out which isn't right & keep the working one & send the duff one back..
[most games will run on 8gb in slot1 [usually nearest your CPU]]
Or send both back saying one is not right and ask for a matched ram replacement.. -i like ram with next sequentially numbered -so its from the same run ..but that's me being old-school.. goodluck but that means pc downtime whilst they sort it