Okay, sometimes there is a hatchling that comes out real small and weak and it just does not have what it takes to make it. It fails to thrive no matter what you do, and its not your fault, it is because genetically the poor thing is just not viable. It is a normal part of nature: weak ones hatch out in the wild too, and they probably get instantly eaten by the strong ones. You know how Nature is, sometimes it really sux.
Ok I say that not to discourage you but to prepare you for the worst. Now, what you need to do is hand feed / force feed it, whatever you can (gently) do to get nourishment into it daily or twice daily. If you can get it past the not-thriving stage, sometimes these geckos will decide they want to live after all and start hunting their own food. If they start doing that, they have a very good chance of making it.
For a very good formula for force-feeding, see the link below.
LeafTail's link to Marcia's recipe