If the black food in mantis belly smells undigested, it might be protein over load and or if the room temp falls below 75F. Try feeding less and sometimes coat food with honey ans pollen powder.
I put the 32oz cups that they grew up in inside the 12"x 12" screen cage, so males may have a chance to escape whilst .. Keep houseflies / hydei available and mist as it encourages mating.....and they like to drink water sometimes.