Runty Tiger Barb

I've got a 10 gallon tank with 4 small albino tiger barbs. I've noticed
that 3 of them are starting to grow. But the 4th seems not to be
growing as quickly as the rest, if at all. I've been trying to see
what, if anything, it's been eating. At least as of last night, it
seemed to be nibbling, but then spitting out the food.

Any ideas on what might be wrong? Do fish suffer from bulimia?
Anorexia?

As of three weeks ago, the tank is fully cycled and water levels are
near perfect. Nitrates are low; partial water changes weekly; water
temp around 78.