I'm having problems with the belly strap on my stacker setup slipping off, and wanted to see if I'm doing anything wrong.
I just put a Malone stacker on my TracRac on my pickup. The bars are huge, and they're ~6 feet apart. In the past, I've only carried two kayaks deck down on foam blocks, but now I want to carry four. They're all 10 foot recreational sit-ins, around 29-30" in width.
I put the first two boats up, with the foam blocks and two straps each (through the hole in the stacker, over the boat, then around the crossbar, as described in the manual). I put a belly strap around the widest part of these two boats and cinched it down, followed by retightening the other straps. I put on the next two boats the same way. I added another belly strap around all four boats and retightened everything again.
That 2nd belly strap keeps slipping off. I can see why. It's at the widest part of the boats, so if anything can jiggle at all it'll migrate toward the narrower parts.