Ahh reptile food.
There are lots and lots of alternatives to crickets.
First.. you can buy smaller crickets, if you are willing to buy them online.
But, other feeders:
Cockroaches (you can buy them online, don't use wild ones because of pesticides)
Silkworms
wax worms
superworms
house flies (buy them online for the same reason as cockroaches)
grass hoppers, if you can catch them in a pesticide free area.
moths, same as grasshoppers.
meal worms
fruit flies, if your chams are just babies
butterworms (I hate them, they stink.)
hornworms
stick insects, if you can find them for sale. They are illegal in a lot of states.
praying mantis- you can buy these online as ova and they will hatch. High mortality rate because they canabalize each other.. but make for a good food.
"phoenix" worms- really larvae of the soldier fly
How old is your Jackson's? What type of set up do you have it in? Are you sure there isn't something else going on that is causing it to not eat?
Heika