First try flipping the polarity on one of the subs.

Next, disconnect one sub and try flipping the polarity on one of the coils till the woofer moves much more one way than the other.

Disconneced the sub you just tested and do the same with the other. Then wire them both back up and listen, if they still arent loud, flip the polarity on one sub. If still not loud, check the signal they are getting and see if you can flip the polarity for your subs on your head unit to see if that helps at all.