Do you have a load? What load value? If you have a 3A load please make the load small. Maybe 100 ohms.
Please try removing C10 470uF to see if the voltage comes up better.
__________________________________________
When power is applied, current from R6, R7 charges up C5 to 5.8V, the PWM starts working, the IC pulls power from C5 and the voltage on C5 comes down to 4.8V where the PWM stops working. Now C5 starts charging up again. When the PWM is working power is moved form the primary to secondary. When the PWM is not working there is no power. I can see the PWM is (working, ----not working----, work, ----no work---) I do not see power moving to the secondary. Why; I can not see from here.
This is a typical 'start up problem' with power line PWMs.
__________________________________________
Look for:
Transformer has winding reversed. (phasing)
Transformer inductance is too low causing the IC to go into current limit.
Transformer saturation.
?????