Amps typically like to drive higher impedance when bridged, that is they will not tolerate low-impedance loads. For example, many amps rated to drive 4 ohms normally, state 8 ohms in bridged mode. Depending upon how you connected the amp to the speaker's voice coils, you may have reduced the impedance and/or it may already be too low for your amp to drive in bridged configuration. It's also possible you hooked it up wrong...
My initial guess would be to make sure you have the voice coils connected in series, in the right order (+ to - to + to -), with the outer terminals connected to the amp.
HTH - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley