Originally Posted by
bikerdoc
Exactly as ShuBen has advised. Most often LED flashers need a resistor installed somewhere along the circuit (in-line resistor) to effectively modulate the current, else the LED act in various ways either flashing too fast (which is the most common complaint) or as you have found, don't flash at all. If you are not comfortable fitting a resistor then head to a automotive electrician, who would be able to fit exactly what you'd need and for not much money. If you try to do this yourself, then you will find that resistors have different ratings (ohms) and therefore would need to experiment a little to work out what you need. A way to overcome this problem is to find variable resistor (potentiometer) in combination with a dimming light switch (like what one can find inside buildings) rigged up to your bike. With this set-up fitted, once you get indicators flashing at the desired rate check the resistance across the circuit using a multimeter and "bingo" you have the nominal resistance for the desired flash rate.
Some LED indicators have resistors built in on the PCB within the indicator housing as ShuBen has advised, though based on your findings it is highly unlikely to be the case with the ones you have bought unless the resistors are the wrong rating for your bikes electrical system.