Originally Posted by buickid
If its a regular flasher relay, it needs the correct load to blink as designed, one bulb probably isn't enough. Thats why you'll see people with blinkers blinking really fast or not at all - one of the bulbs is cut.
What he said.
A "regular" flasher has a contact that bends when it heats up. When it turns on, it needs enough current flowing through it to heat it enough to cause it to bend. After it bends and turns off the lights, it cools and springs back to turn the lights on again. If the rest of the circuit (the signals themselves) don't use enough current to cause the flasher spring to heat up, then it remains on all the time.