One issue I can see; if you are detecting the zero crossing referenced from the transfomer secondary, there is likely to be a slight phase difference.

Also, is the objective of the circuit to adjust the intensity? If that is the case, why use PWM? That is likely cause a lot of trouble, and unlikely to work. All you need is a fixed pulse (say 200 uS); depending on how much after the zero crossing the pulse is given, the triac will control the bulb's intensity.

Regards,

Anand Dhuru