Hoover WindTunnel Self-Propelled V Belt

I replaced both belts and still the thin V-belt is loose??? Is there an adjustment that I am missing? Hoover Self-Propelled WindTunnel Bagged Vacuum U6441-940

The v-belt started flapping / knocking against (very loud rat-a-tat-tat) something after I replaced the flat belt. My wife ran it that way ("It's loud, but it's picking up REALLY GOOD!") long enough for the v-belt to begin wearing a groove past (at the end of) the groove it's designed to run through on the bottom plate.

I bought a new v-belt because the old one did have a crack in it and seemed loose (positive it's the correct part: Hoover Vacuum Belt #38528-034) and positioned it in the groove to the inside of the large (flat belt) pulley in back and in the groove on the brush roll. The new belt seems too loose, also. I can't find a diagram to compare, so I'm not sure if the v-belt is positioned correctly. The Owner's Manual is worthless for this.

When I release the handle from upright, the v-belt is pulled (kind of) taut and seems to make contact on the outside (writing side) with the little plastic roller thing under the brush roller (adjusts up and down for carpet height - idler arm?). But it's not taut enough to keep it from slipping and flapping.

The brush roller turns when the handle is released down, but the v-belt is whacking against something and continues to grind that new, extended groove.

I tried pulling the v-belt around the outside of that pulley / roller thing that's under the brush roller and nothing would move at all when I turned it on. It was nearly impossible to remove, so I assume that was NOT the correct installation.

I've had the thing apart so many times that I've stretched out the flat belt and will have to replace that again, too.