Hi bigbug,Welcome to the forum.
It sounds like you've put all the crickets in with her at once, which is a bad idea. She can't eat them all once, and the remaining ones will go after her when they get hungry. Only put a couple in at a time, and wait until she eats them before putting just another one or two in. Spread out the feedings to only a couple of crickets a day or two, depending on how her appetite seems to be.
Keep your crickets in a different container, and feed and water them as well while they are waiting to be food for your mantis. The crickets you get at petshops usually aren't fed very well (if at all), and it's best to feed them up nutritious food so they stay alive until you need them and your mantis gets the benefits of the nutrition. Put some oatmeal, cat or dog food in with them. Also give them fresh fruit or veggie scraps like potato, carrot, or apple. Dandelion leaves that haven't been sprayed with insecticide are also a great free and fresh leafy green to give them.
Don't forget to water them too. A shallow jar or cup lid with wet cotton or paper toweling in it works well. Don't put a bowl of standing water in there or the crickets can drown in it.
I'd continue hand feeding her if she has problems catching her food. She is likely near the end of her natural life cycle right now, and nothing you do can bring her back to being good as new. But you can help her and take care of her needs until she passes on. You might consider keeping the ootheca and either placing them outside, letting nature take its course, or keeping it inside and raising the youngsters. See the forums section on
Breeding and Nymph Care . You should also search and browse the forums here for lots of great information and tips. Good luck!