The difference between 21 W and 23 W isn't enough to stop a flasher unit from flashing.
The current variation due to the change of battery voltage with the engine speed and the variation from one bulb to another is much bigger than that.
Have you fitted LED bulbs that say they are equivalent to 23 W? If so, they probably take far less current, maybe only 0.25 A instead of the 1.75 A that a 21 W bulb takes at 12 V. That would stop the flasher unit from flashing.
If that is the case, you need a resistor of about 4 Ω, rated at 50 W, in parallel with the bulbs on each side of the bike.
You might also try a car flasher unit as they work better than motorcycle ones. The car ones flash more consistently, they start at the start of the "on" cycle not the start of the "off" cycle, and they flash twice as fast when one bulb is blown.