HWs placed on the screen is a good idea... That will get teh cham's attention... Or on a stick/vine in the cage near by.
How big is the HW?
Trust me, once your cham decides "hmmm, I'll give it a try" He'll never hesitate again, unless he is full.........
If the HW is large, almost too big for the cham, be SURE to smash the head of the worm so it's pinches can't bite the cham. HWs don't go without a fight. I have had my melleri get HWs stuck to their lips... HWs swinging around vilently... and People reportting of HWs biting once inside the mouth.
Now... This doesn't mean they aren't good feeders... Just means you need to feed the right size and take the right precautions. I don't feed HWs without watching the cham completely finish the worm. I don't leave a work out and walk away.