I hand and feeder feed my dubia. When I hand feed I use a very small cup they can't climb out of and spin them around until they are on their backs and offer them that way. I think the waving legs attract attention. When I put them in the feeder I put crickets or a super worm in with them to stir them up. When they sit still I think they look too much like rocks. They have to move to be appetizing.
As Virgil1972 said above if your cham eats one when going after something else he might get a taste for them.
Good luck.