It sounds like the mantis may be nearing the end of its life. But, you can force feed a mantis by gripping it behind its "upper shoulder area" and when it fights, put a piece of meat to its mouth with tweezers or with a toothpick. Once it bites the meat, or an insect such as a mealworm, and tastes it, it will probably grasp it and start eating. Also, mantises learn to drink water out of spoons once they taste the water in it. Mealworms can be bought at pet stores. and are easily raised in oatmeal with a sliver of apple in the oatmeal for moisture.