They'll start respawning later in the game. I'm not sure of an exact point, but enemies seem to respawn whenever you leave an area. I think they also add a couple of enemies for the sake of getting these blitties.
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Walk up to a toothy (BUT DON'T PUNCH IT, unless if you won't give it a 1 hit KO). If it has a fat goomba with it, attempt to inhale. If a blitty pops out, keep sucking. If not, let it ride and beat the snot out of them.
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