Some the batwings are not amplified. Look for the plate as shown by hande9's link. It induces power on to the coaxial cable. The amplifier is actually in the antenna head. Check the coaxial cable output on the power supply for 12 volts. If present got to roof. No 12 volts the power supply could be bad.
The small red light should come on if power is getting to the antenna amplifier. If it does not come on, remove unit from the wall. Verify that there is 12 volts dc to the power supply. If present, go onto the roof, disconnect the a coaxial cable from the head. Check to see if there is 12 volts DC on the coaxial cable. If yes, head is bad. If no, you could have coaxial cable problem.