injectors get dirty/blocked due to dirt/impurities in fuel, or even if somewhere gap for air/dust.
1- there is one spray that is sprayed at high REVs, one I got but not used was WARCO carb & choke cleaner.
2- Those can be opened and cleaned by mech.
3- there is one additive for fuel tank, that is added to petrol for cleaning, powerful one I think is STP in black colour bottle.