How much are you putting in? What else are you feeding the tank? How many times have you tried the jello?
Sometimes they just need to recognize a new food as new food. I feed them every other day or so. I also use about a half postage stamp size for a handful of large brigs. The smaller snails may not need to eat as much as you are feeding, or may be feeding on algae, biofilm, leftover foods etc and so are just plain not hungry. My nerites rarely eat the jello, unless the algae and biofilm are "depleted". What type baby food are you using? Another thing you could try is to add a few drops of honey to the recipe, but I think you probably need to try adding lesser amounts of what you have made a little less frequently. Give them a chance to recognize it as food