2012 Kia optima miles to empty not going up

as it says for some reason starting last night the Miles to empty isn't going up at all even though the bars says it has 3 full bars and I put in a total of 30$ since last night the Miles to empty is going down but not up for some reason and I don't want to take to local mechanic if isn't easy fix and isn't going to cost me $75 for them to do a diagnostic