Illness/Sickness: Eating troubles

I have currently have two younger axolotls (about 7 months old and 7 inches each) in an established 20 gallon long tank with a fine sand substrate. Toothless, my melanoid, is acting normal. He's hitting his food rather aggressively and eating as much as I'll give him, as always. However, Hiccup, my leucistic, has stopped acting like this. His activity levels are the exact same as before, and there is no fungus of any kind on him. He's not showing any obvious signs of constipation or impaction, and as far as I know, he's still pooping as usual.

I normally feed them earthworms, but have recently added axolotl pellets from Flymeat.com to their diet for some variety. I went on a 2 week vacation to New York and had my mom feed them only pellets (every other day) in the time I was gone. They both ate wonderfully in this time - my mom said Hiccup wasn't snapping with too much aggression, however he's always been on the gentler side, so this wasn't concerning to me.

The day after I came home, I fed them each a large earthworm. Toothless ate like normal, but Hiccup gave me some trouble. After the first two or three pieces, he would barely snap and would just hold the food in his mouth. He has been eating his normal amount (up until today - he ate one less piece), but not without a lot of extra effort. It just isn't like him to ignore food like this, and it's concerning me a little bit - especially since Toothless is acting fine.

The water paramaters are fine, their temperature is at about 69° farenheit - a touch warm, but it was at this temperature when Hiccup wasn't giving me a hard time.

Can anybody provide some insight on what could be happening here? I really don't want my baby to be ill. It may be nothing, but I'd rather be safe than sorry.

Just a small update, Hiccup is snapping with even less energy now - when he even snaps at all, that is. Does anybody know why he might be doing this? His mouth isn't swollen and there are no visible injuries. This isn't like him and it is the most worrying part of the situation for me.