A picture is worth a 1000 words for this one.
Did you have a picture of the routing of the old belt?
I can think of 3 things that cause this.
Belt is not routed properly between all the pulleys.
There's a tensioner spring that's not connected to an idler pulley.
There's an arm that's connected to that tensioner spring that needs to be turned to a location to put tension on the spring.
Do you have any pictures or schematics?
You want to make sure the V portion of the belt goes into the grooved pulleys (i.e. the spindles).
The flat (back) part of the belt goes on the wide, flat pulleys.
Then eyeball to make sure the belt isn't touching anything metal. If it is, the belt isn't routed properly.
Check the idler spring to make sure it's attached.
Where it attaches to the tractor at the clutch, if there's more than one pulley (of different sizes) and your belt is loose, put it on the larger pulley.
Does this look like yours?