I just bought some Large black crickets and the mantis is ignoring them. One even ran up the rock face and he just run away from it. Maybe they are too big for him?
If he is ready to molt then he may just strike at the cricket to scare it off. Remove all food and wait. If he hangs there not moving and looks swollen then he is going to molt very soon.
Just went back to Pet shop and got some Micro brown crickets, he is not interested in them either, I even tried tweaser feeding him so I guess he is due to moult.
I got him on Friday, not sure how old he is but he is only small, about an inch and a half in length and has a really small head. Here is a pic I just took of him