Richard
The old style flasher units turn on and off by heating up inside due to current flow thru the flasher unit. Opens and closes the circuit. The LED flows so little current that the flasher doesn't heat up properly. Also why when you attached a trailer to a car they used to flash really fast until you installed a heavy duty flasher.
The 2 solutions already mentioned or you can add a load resistor (guessing 6 ohms 50 watt resistor) to the circuit.
Steve