If R2/C1 time constant is low enough to permits cycle at 50Hz , i think you could try to lower R1 to increase current of Photodiode.
With 220V you have only 3mA max that flows on input circuit (maybe photo transistor is not even ON).
You could try with 15K (about 15mA max). (pay attention to power dissipation on R1)
Have you put a scope on the input to the gate? It may be that C1 is not discharging fast enough through the transistor to drop below the gate threshold before the next cycle of AC turns the LED back on. You could try a smaller capacitor (or no capacitor?).