Just spit-balling here, but is it not spitting because the prop is pulling the exhaust and water out of the leg? The holes on the back side of the lower cowl are just idle relief holes, that will spit water out, but not really a great indication of water flow. I've never checked it while in gear so I don't know how much water comes out of the idle relief holes while the prop is moving, if any at all, I just check it idling to make sure it's spraying some water out.
Someone did a write-up on adding a telltale to the small Forces here on the forums. I've been thinking of adding one as it's a good indication of water flow, but never got around to it....