Hey, I raise dozens of hatchling diamondbacks every year so hopefully I can be of assistance.
First question I have is how old of a hatchling is it?
I normally don't offer food to my hatchlings for a few weeks after hatching because the rapid absortion of their yolk sac after pipping the egg leaves them with a full supply of nutrients for several weeks or even months.
Secondly, are you feeding in the hatchlings' setup or in a separate tub?
I would begin by feeding dried gammarus shrimp in the hatchlings setup first, and then adding Tetra Reptomin and Gammarus Pellets to the diet after it has begun taking the gammarus. Only feed one of the foods at feeding, not all three, or else you might get a picky eater on your hands that only takes his favorite. Basically at feeding time, just give it one choice. After it has started eating regularly, alternate the different types of foods. After about a month or so of your hatchling comfortably eating in its environment I would switch to feeding it in a tupperware tub so you can control how much it eats and it keeps the setup much cleaner.
Thirdly, what type of setup do you have?
I would have only a few inches of water, a basking spot, filtration, and lots of plastic plants for cover. At this stage young terps are very secretive so need to feel secure. After about 1-2 months you can increase the level of water, but still keep a good amount of plastic plants. You could use real plants if you want, but it is a lot more maintenence, and DBT are primarily carnivorous, so why the hassle?
Fourthly how is your lighting/timing?
Always use a UV Bulb (ZooMed, etc.) I start them out with 75 watt bulbs, and switch them to 150 watts after a couple months. This wattage also keeps the water at about 80 degrees during the daytime. I keep mine on 14 hr daylight cycles to mimic summer when they are at their highest metabolism. 7am-9pm
Fifthly, is there any observable medical complications?
Look puffy eyes, lameness in limbs, nasal discharge.
I'm pretty sure your hatchling just isn't in the mood to begin eating yet, they never are. Just give it time, try feeding the dried gammarus shrimp every day in small amounts. Keep your distance during this period so the guy feels comfortable, but observe to see if it does begin eating. Good luck.
-JG