there are afew ways i haver tried.
cutting the veggie into a small sliver and either trying to get your cham to thinks its alive by wiggleing it or rub alittle juice from it on thier mouth, i think they smell it and gets them going.
the other way i have tried with success is to finely chop up some veggies and put them in a feeder cup with the feeders, your cham may get some caught up on thier tounge when shooting for an insect and see what hes missing.