Would suggest replacing both bulbs on the faster side (front and back?), to rule out a faulty bulb, first.
Check the specification of the bulbs on the "good" side, so you pickup a matched set of new ones. Best not have mixed rating bulbs.
I've replaced the original bulb indicators on my Bike for LED indicators, also adding an electronic Flasher Relay that will work for LEDs OR bulbs. (it also means you don't need any Resistor harness attached inline with the new LED indicators (cleaner wiring).