As Ridgebax1 states, it is all about movement to trigger feeding. Crickets move on the screen, and if they don't get eaten generally will cluster around the basking light for heat. The best screen climbing worm is super worms, place them on the screen below, and near, the chameleon, he will strike.
If you have a small chameleon, order small super worms from Mulberryfarm.com, put them in wheat bran, and keep a fresh slice of apple in the bran, and they are kept at room temperature. They will grow with your chameleon.
CHEERS!
Nick