VIPER17 Startup problem

We are using VIPER17 in our products as a standby power supply for a couple of years in mass production. Recently we faced a problem that if the mains is off and on again this power supply can not start-up. Mains has to be off for a long time (more than hour) for this power supply to be able to start-up. The problem is not related to the heat, input voltage or load conditions. Replacing IC with another from old stock fixed this problem. Increasing the value of C

DD

(

to pin 5)

also fixed this problem. Our tests show:

-

after mains was OFF for a long time (>2Hr), discharge current I

DDch

was 2.4mA and unit started successfully .

-

if Mains was OFF for short time (but bulk capacitor is still charged) , discharge current I

DDch

was 3.43mA

and unit could not start.

-

if Mains was OFF a bit longer time (bulk capacitor was fully discharged) , discharge current I

DDch

was 2.95mA and unit could not start.

Any suggestions why the value of

I

DDch

is changing?