Honestly, it's just the time of year. I have a lot of animals that aren't eating right now, and a couple juveniles that have been skidding by on one mouse every month. Juveniles are even worse than breeding adults, I swear. You said 400+grams? What's your largest snake? How long have you had them?

Their appetites should start picking up pretty dramatically in another month or two. By July they'll all be gobbling down multiple prey items, regardless of where or what you feed them. By August you cand start offering them lightbulbs and plushie toys, and they'll go for it. (Note: do not do this!)