What causes a troy built lawn mower transmission to slip or not hold gear when going downhill?

I had the same problem with my yard man machine. It just started slipping one day going down hill and had been doing it ever since.

What I tried - Checked/ replaced drive belts using OEM replacements (there are 2), no joy.

Checked all Pulleys including variator, All good

Opened rear end, all gears good

Finally I adjusted the linkage from peddle to variator to tighten the front drive belt.

That changed things slippage went away and the machine pulls like new.

But had to adjust it all the way out to handle the slippage.

I am planning to buy a belt from Tractor Supply by length and get it about an inch shorter to regain my adjustments.

I believe the Front OEM belts are a bit too long for the mower as such the belt on the mower works great till it gets stretched and the machine begins to slip and continues after belt replacement. Hope this helps