Could someone please tell me why the TRIAC is not triggering as it should? After zero crossing is detected, timer has programmed delay from 1ms to 9ms, set by potentiometer. The trigger signal is correct, i think, yet the TRIAC's output is not getting chopped as in phase angle control. 
Answer
You need 50mA to trigger the triac (100mA in QIV), and you appear to be getting neither. You should avoid QIV, the triac is much less sensitive in that quadrant.
Your drive signal is inverted, it should be low to turn on Q4 to trigger the triac, so if there was sufficient gate current available the triac would remain on continuously.
It may work when you fix the gate drive, for half-waves only, because your continuous drive is being limited by R7 (100 ohms) and thus C1 is not helping. But there is still not enough current to reliably trigger the triac in QIV, so re-think your design there. Negative gate drive is better.
Remember, always read the data sheet through to the end. The datasheet mentions 50mA on the front but the fine print tells you it applies only to QI, QII and QIII (and only with the junction at room temperature, if you want it to work when cold you need more current).
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