pokes belonging to slow/erratic group have lesser exp than required for their current level. So once u faint few pokes, that gap is bridged and exp bar will start working normally. Until then the bar wont increase but the poke will continue to gain exp. U can check it in the value of exp by opening the poke profile
In short, just faint more pokes, u can observe that the exp is actually increasing, if u check the exp value in the profile