Ditto that. I've found the same thing -- it fills faster when the boat is moving. I've found that sometimes when I can't get the pump started, if I get the boat up on a plane with the valve open and pump on, it usually is enough to get it started.
Adding to your comments, Krejciman:
My routine:
1) Open valve, push ballast fill button.
2) wakeboard
3) Close valve, push drain button.
I leave the valve open when wakeboarding, because I've found the sack will drain partially if you close it -- the sack needs the constant pressure coming in to stay full.