When a chameleon is digging don't let her see you watching her. As you've found out, they usually stop (figuring that its not a safe place to lay the eggs). If this happens often enough, the female may become eggbound.
The substrate in the container that she is digging in should be moist enough to hold a tunnel and deep enough for her body to be completely in the hole when she is finished digging it.
Has she been digging more than once?