ne555 monostable
The most likely culprit is the capacitor's leakage current, since it sounds like you are using an elecrolytic (22uF). The leakage can be about 1uA. All this current, plus the current that goes into the NE555 comparator input, must come from Vcc through the timing resistor. Since the timing resistor is large, the voltage drop across it can become significant and the voltage across the cap never reaches the high threshold. (Actually, it sounds like it's right about there, but there just isn't sufficient current left over to make the 555's comparator trigger. The transistors in the 555 comparator need some current to flow itto their bases, but what's left once you subtract the cap's leakage current is too low.)
To test the theory, reduce the resistor. If all works well, then the solution is to reduce the timing resisto and increase the timing cap. That is because the leakage current of a larger cap will not be much larger, but since the timing resistor is lower, things should work just fine.