What happaned when the flyback transformer is in saturation?
If the core saturates, the CS pin will exceed 1V and cause the controller to latch off after 15ms or something like that, until the power is cycled.
You can monitor the voltage on that pin and see if that is occurring. Also, measure the primary inductance and see if it is as designed (and double-check your design), preferably under bias if your LCR bridge allows that.
Make sure your transformer air gap was calculated and manufactured correctly.