If an inverted pleat in a skirt hangs too open at the hem the most likely causes are insufficient hem width and insufficient pleat width.
Let out the side seams and pleat edges at the hem as much as possible, but if the seam allowances are too narrow for this to have any effect, the appearance of the skirt may be improved by hollowing the front waistline slightly. This lifts the edges of the pleat and causes them to hang closer together.
A similar problem may be encountered with box pleats and knife pleats but it is more obvious when it occurs with an inverted pleat.