South American have electric sense, but not any of the Notopteridea (all clown knife species, ABK, etc) ABK may have them but I don't think so. My ABK aren't very aggressive eaters, but if need be they will eat more aggressive. When my senegal bichir was around, he would eat all the food if the ABK didn't hurry up, so they were very aggressive eaters then, but now they are mellow. They also prefer to eat in the dark.I wanted a knifefish in my future, but I've read that a lot of the other fish I like will out compete them.
Don't knifefish have an electric sense (probably like the lateral line) that lets them find food in the dark? Maybe it will eat with the lights off?
Also, check your water parameters. Some species are more sensitive to worsening water conditions and one of the early signs is low or no appetite.