Depending on the type of nem you have or are feeding, getting them to respond quickly may take a little finesse. My Rock flowers I sometimes have to hold the food and wiggle it around a little bit on their short face bumps to get them to respond. My RBTA I just have to place the food near the mouth and it goes to work on it instantly.
You can try using a large water bottle as a feeding shield. Get one that's a little larger than the nem, cut the bottom off and drill some holes in the side for water flow. Place the food for the nem in it near the mouth and move it around to trigger the feeding response. Once the nem grabs hold, place the bottle over it and wait for it to finish eating. Sometimes they get a little finicky and don't want to eat right away. This usually happens for me if I go to feed them too frequently. Such as every 2 days for the bigger rock flowers they tend to be a little meh at feeding. If I wait another day or two I will get a much more aggressive response. My RBTA on the other hand will happily eat daily provided it's smaller pieces of squid, silver side, or scallop. Anything else like krill it may reject.