LEDs tail lights often do not pull enough current to trigger a standard flasher circuit.
you can try a automotive tail light bulb in place of the LEDs to confirm this is indeed the problem. if so, you can wire a resistor in parallel with the LEDs to increase the load on the flasher.
link resistors are down about 7/8 of the page. on that same page you'll see flashers made specifically for LEDs. these require no reistor.
i lucked out on mine... opened it up and found a pot that needed turning to lower the current threshold.
-sj