Curse on Hit Doesn't Work in Headgear?

Cast when damage taken triggers are working fine for me. Make sure that your gem levels aren't too high, the level of your cast when damage taken gem is important, the lower level it is means it can only trigger lower level supported gems, by the sounds of it your supported gems may be too high level. You can find more information about this if you read the CWDT gem.

Happy to help. That's odd, I tried it with the exact setup you listed (Great Crown, same gems / levels) and it works just fine for me locally. Have you tried removing other gear to see if this still occurs? In cases like this we need to narrow down what the cause could be as it could be a different piece of gear, a passive point, etc.