Some of the single, angle-cut pickup, top-cap covering showers have been shown to not direct a lot of water to the drive. Was curious if that was the type you were running.

I've heard that one of the best ways to do it is to run a dual stage water pump/pickup, use one for the engine and the other plumbed to dump on the drive.

On my BMax's I can see water flowing thru the drive just off idle speed. The upper is completely water jacketed, I bet this drive is very similar.