The jet, which is what outputs the fuel to the flame, can sometimes become clogged with carbon deposits that build up over time or with debris. Several stoves, such as the popular MSR Whisperlite XGK, have a self-cleaning shaker jet so all you need to do is turn the stove upside down and shake it. If your stove does not have a shaker jet, take a pin or paper clip and poke the debris out of the jet spout.
Occasionally, the fuel line that runs from the refillable fuel canister to the stove can become clogged, resulting in a low flame or intermittent flame. To clean the fuel line, unscrew it from the fuel canister and use the cable in the line like a pipe cleaner to clean out debris. Reattach the line to the fuel canister, pressurize the canister with a few pumps, and run fuel from the canister through the line to flush it.
If you get a persistent yellow flame after the stove has warmed up, the fuel canister may be overfilled. Turn off the stove and let it cool, then empty some fuel from the canister and reattach it.
The fuel pump in the canister used to pressurize the stove. The rubber seals sometimes get dry and need to be remoistened. Remove the pump plunger mechanism and squirt a few drops of Pump Cup Oil into the pump cup. Reinsert the plunger mechanism into the fuel canister.