make sure that everything has a secure connection. if the positive or negitive slipped off it wont work. so recheck everything and make sure nothings come apart
mostly ide look at the wiring between the amp and the sub. make sure its plugged into the right spot on the amp and everything is wired correctly depending on the voice coils and ohm configuration. make sure everything is still conected. if it still doesnt work ide call the company u got them from.
if on of them works and there both wired the same and connected to the same place it must be a bad sub
The sub that's not playing as hard probably has a blown coil. If you had coils in parallel and you blew one, the sub will then have more resistance (double) and therefore have less excursion. For both of you guys, you need to check if your coils still have resistance - if not, they're blown.
Are the subs hooked up to a mono-block amp (1 channel) or a 2 channel amp. If it is a 2 channel amp, check the rca connection between the head unit and the amp... If thats all wired properly, check the settings on the head unit (fade/balance)...
Try laying the sub on a flat surface and pushing the cone in. if you feel a scratching vibration then your sub is bad. do this with your fingers directly in the center of the cone because if you do it at an angle you will prob feel scratching even if the sub isnt blown.