The first thing you have to figure out is why they're losing weight. Some geckos do get tired of a certain food item for a while but illness, ovulation, and improper husbandry can also keep them from gaining weight. I personally never use waxworms, some geckos are almost impossible to get back on "real" food after they get used to candy treats. Superworms and roaches are both great choices in addition to mealworms and crickets.