UCC28600 based 20W SMPS with 600VDC input

There may be many reasons why the converter isn't turning on.  You need a small filter cap on FB (up to 390pF) to prevent noise, a small filter cap on CS (100pF may help), layout is critical as is transformer design.  You need to look at the waveforms to see what OVP, VDD, CS, SS, and FB are doing.  Use the design calculator used to select Rcs, Rpl, Rovp1 and Rovp2 and also to make sure the transformer has the correct inductance and turns ratio.  Refer to ww.ti.com/lit/an/slua399b/slua399b.pdf  for other considerations when using this controller.

first I would check to see that, when the input voltage is applied, VDD rises to 15.3V (max turn on threshold), FB should go to 5V and SS should charge up.  When SS gets to 5V the OUT should start switching and FB should go from 5V to the appropriate level determined by the control loop.  If FB stays at 5V, SS is pulled down to ground, and VDD doesn't stay up above 8V then you won't get any switching.  Start at no load if possible. 

Did you use the design calculator?  The combination of the Rcs, Rpl, and Rovp resistors is critical as is the transformer design.  You need to make sure that the design is within the operating parameters of the controller (on-time, frequency, OVP thresholds, etc.) and the design calculator helps there based upon the input voltage and output requirements.

I just noticed you do not have a small (0.1uF) ceramic decoupling capacitor on VDD...this is required (non-negotiable, sorry...put it on there as close to the IC VDD pin and GND pin as possible...solder a leaded cap right to the IC right now until you can re-lay out your pcb...).

Remove the 220k resistor from STATUS...you aren't using it for controlling the bias to a PFC controller and you don't have it configured for audible noise reductio so it should be open/floating.

Did you add the filter capacitor to CS?  and one to FB?  noise on these inputs may be the issue.