1. Check your wiring. Make sure nothing is being shorted. Even a single strand of speaker wire shorting will cause the amp to go into protection.
2. Make sure you're not exceeding the impedance (ohms) of each channel of the amp. Most 2 channel amps can only handle 2 ohms each. If you bridge then that usually drops to 4 ohms. Some mono sub amps can go as low as 1 ohm. Check the specs of your amp.
3. Check for correct polarity to the speakers. Reverse polarity will cause low bass output.