Same as most cars and trucks you see on the road that are turning, only to see the turn signal stay on bright. Most always is a burned out bulb on the side that is not flashing. Most standard flashers work on the resistance method. Usually a bi-metal strip that completes the circuit to ground. The bi-metal strip is usually two layers of metal bonded together, they can be steel and copper, or two other metals that expand at different rates when voltage is applied and they heat up. I.E. copper expands faster than steel, but when they are bonded together, it will bend the metal. When you apply voltage to the strip, one metal heats up faster than the other, causing the strip to bend and make contact with the other part of the circuit, completing the circuit. A bi-metal strip that bends with increasing resistance (or heat, in this instance). If there is not enough resistance, the metal strip will not get hot enough to bend and start the flashing cycle.
All flashers are designed to work within a window of 'heat' or load (number of bulbs).
Since you do not have the original number of turn signal bulbs to 'heat up' the flasher, the bi-metal strip does not get hot enough to bend, therefore not making contact to 'flash' the bulbs and they just stay on and do not flash.
This heating and cooling of the bi-metal strip occurs very rapidly, heating up (bending the strip) and cooling (unbending the strip) that makes contact and releases contact of the circuit.
This heating and cooling action of the strip is actually the bending and unbending of the strip that actually makes the 'clicking' noise we all hear with the flasher.
That is why when you convert to LED lights, there is not enough resistance to heat up the strip to make it bend and make contact to flash the lights. This is why a module or electronic controller is needed to make the flasher work correctly or you have to use a solid state controller to flash LEDs.
If this sounds confusing, PM me and I will try my best to explain in more simple terms how this works.
Loud pipes save lives...Nah, they just sound cool.
2009 Honda Rebel-Ultra Blue, the fastest color. (Now residing in Illinois!)
2012 Harley Davidson Sportster XL1200 Custom.