Feeling like a total newbie, we cannot seem to get suction started to empty either holding tank. The tanks ate nearly full as we can look in and see slurry but I'm not sure if there's a drop-in stanchion pipe or some other item I might be missing. There's just a big air gap to look at, and while the pumpout hose does apply suction, it won't draw anything but air. We primed the pumpout hose, made sure there's a good fit but nothing happens. I have to be missing something basic but have had a few other dockmates helping out to no avail. Thoughts?