hey, i had that same problem and it turns out that it can be a number of things, first off maybe your amp cant go into a bridged mode in mono, maybe it can only do it in stereo, maybe your amp just cant push that hard, or maybe your amp just isn't built for pushing subwoofers, maybe its just made for mid's. or maybe u have a lack of power and u need a farad? it all kinda depends maybe u need bigger guage power and ground cable, check into all these things, oh and check how u have the subs wired, parallel of series, cus the ohms might be wrong,