Drive belt is slipping on a pull. Replaced with Cub Cadet Belt. What else could be wrong?

Most likely its a belt tensioner spring that attaches to a pulley somewhere in the belt "loop" or "path". Look for a smaller pulley that is attached to an armature. That's usually the belt tensioner. If its not that check

no adjustment when worn replace

Because I didn't know they used 2 belts in the diagram on parts tree.com . I bought the 79" belt and not the 78" belt.

In most cases the hydrostatic drives are not made by the company tractor or lawnmower.
This is in the owners manual.

Hydrostatic Transmission
The hydrostatic transmission is sealed at the factory
and is maintenance free. The fluid level cannot be
checked nor can the oil be changed. Always keep the
area around the transmission cooling fan free of grass
and debris at all times

Cub Cadet LT1024 Operator Manual

How to Change the Hydrostatic Fluid in Cub Cadet Lawn Mower

https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1GGGE_enUS385US385&ion=1&espv=2&ie=UTF-8#q=cub%20cadet%20hydrostatic%20fluid%20change

I hope that this can help. Tom

you need to change thr hydr fliud in the transmission and the filter

the belt tension could cause the mower to not pull on a hill you can pull on the belt and if it's flopping around also look at the belt for signs of oil oil leaking on the belt will also cause the belt to slip

is the drive belt slipping?

they changed the part number for that belt new # is 954-3068 B x 110.6 your old belt was probaly streched so when you install new one it should ride on pulleys correctly

manuals.mtdproducts.com/mtd/GetFile.do?filename=769-02100.pdf

Check your mower deck spindles and belt. If belt is slipping at all the PTO will not engage.