Phase Cut to Control Speed of AC Induction Motor with Arduino
The problem is that the Triac turns itself off when the current drops. Giving that the motor current is out of phase in respect to the voltage, and probably by 90 degrees in your case, when you activate the Triac near the middle of the semi cycle, the Triac suddenly turns off because the current drops. You should keep high your drive from the intended point until the end of the semi cycle -OR- you could emit several pulses from the intended point until the end of the semi cycle.

Doing so it is more difficult to turn off the Triac just before the next (semi) cycle, but it is necessary in order to use the full semi cycle.