I would increase the frequency of your cricket feedings, once every 3-4 days is rather infrequent. Alternate between the crested gecko diet and crickets, feed t he diet one day, dusted crickets the next, the back to the diet etc. This will help your geckos to grow faster. Also, are all the small ones housed together or are they in separate containers? How big are the containers? Sometimes a too large container does make it harder for them to find their food. Make sure there is alot of cover (ie alot of fake plants to give them more security). If kept in a group, are they all growing at the same rate or is one or two larger than the others? If some are larger, take them out and give t hem new homes, so they are not stressing the smaller geckos.
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PHLdyPayne