Bulbs maybe fine but it's possible you have:
1: Corroded fitting in a bulb holder.
2: Most commonly with a disco, it's the plug slipping off the holder on the lights in the bumper if you have a 300 series. Put hazards on, go to the rear bumper indicator and push on the fitting (wire) and you'll find it'll work.
3: If you have fitted LED indicators on the right, it maybe because it's such a low drain used it causes the relay to flash fast as though there was no bulb.