different dragons grow at different rates, the missing tail has nothing to do with growth. To be certain of his growth rate he should be weighed and measured weekly. Even with good bloodlines it is possible to get a runt that will not grow. It is not uncommon with inbred animals (that's why you buy from reputable breeders). Make sure you have good UV lighting. Not just any UV. Different bulbs put out different amounts of UV at different distances. Supplement with D3 and give enough food for the animal to pig out, don't skimp. Make sure the Cage temps are correct too.
Sometimes you just get a slow starter.
good luck and happy new year
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Bennett

www.beardiedragon.com