Hey,
I would relax and not begin to worry this early in the game with your newly acquired charges. The most obvious symptom of what could be happening is that your non feeder, is going into a shed. This is typical of bloods, and particular with most juveniles. Wait for a few days and resume feeding trials with him or her, if the obvious signs of shed are not apparent to you. I.e., opaqueness. I have a few gluttonous juveniles and even adults that will feed during this period, and there are others that steadfastly refuse food until after they have shed. If your set up and all parameters are within appropriate limits, then the aforementioned is probably your culprit. If however, there are no obvious signs of shed, then maybe changing something in your set up might be your answer. There are no grey areas with bloods, as they are quite the fussy critters and like very much what they like and that's that.
They are ambush predators, and enjoy a good hide, and I am not necessarily speaking of a hide box, but a cypress mulch. A substrate like cypress mulch, allows them to burrow and hide beneath the top layer, and gives them the security and ambush advantage that they would normally find accessible in their natural habitat. So providing them with something other than newspaper or what ever you're using might just be the key to eliciting a positive feed reaction and response from a shy blood. Also, if you're feeding frozen thawed rodents, having a small container filled with warm water to heat the mouse in can also, help in eliciting a strike response as the heat gives them the heat signal necessary for identifying the item as food. I find that when I don't get a strike from my feeding short-tails or bloods, that warming their food immediately changes their lacklustre interest and they immediately strike and constrict.
Remember to feed with a long pair of tongs or forceps. I use a pair that is at least 12 inches long. I keep several around for different feeding sessions with different animals requiring different needs and tactics.
-Angel
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In life you can fall many times, but you're only a failure, if you don't try to get back up!
Evil Canevil