Why don't some of them spit fireballs?

Is that a bug in the game, or are some of them not supposed to spit?

It isnt a bug in the game. Its intentional. The fire from the seahorses (Lindas as we call them) is a set pattern upon entering the zone. If you up a back to the start of the zone the patterns of the fire will most likely change. This is the same for boss patterns. When doing an up a or dying the boss pattern will change.