Yes - there could be a mechanical reason for fish taking in food, but spitting it out. One common cause is an obstruction. No way to treat that though. Very often, when there is an obstruction though, the fish sort of "gives up" and stops trying to feed at all. Since your fish is still eating nori, that seems less likely to me.
Do you have access to other types of algae? Some stores sell live algae. There are also the Two Little Fishies" brands of Sea Veggies dried algae that are more nutritious than regular roasted nori sheets. Finally, have you tried gel foods that have a similar consistency to nori, but are higher in protein? Repashy or Omega 1 are two types here in the US. I use Mazuri gel diet, but that is more for public aquariums, I'm not sure they sell smaller amounts.
