A thin tail means that the gecko is not getting enough food. This can be due to a lack of food availability (not feeding your colony enough), or bullying from other geckos who are higher in the colony pecking order.
Make sure you are offering crested gecko diet 3 days per week and insects 2 days per week. If you are, the geckos are probably competing too much for food. Add a second or third dish of CGD (depending on the size of your colony) in a different location, and offer more insects per feeding.