Welcome to JUST ANSWER small engines, let me try to help you !!
The tension spring is all that tightens the belt. Is it in the correct holes? Does it put good tension on the belt? Or could the bracket for the idler be bottoming out on something, not letting spring pull the idler tight against the belt? Has the spring been checked against a new spring? The pedal linkage is not holding the bracket from putting more tension on the belt?
It puts good pressure on the belt and is not bottoming out on anything. I've wondered if the transmission could have some sort of problem. Could it be low on fluid? Surprisingly this has been an issue for about four years now and it slowly seems to get worse. Some days it will buzz right up the incline by my home and then the next time I use it, it will not go up the incline. It's not a severe incline.
The spring seems ok for putting tension on the belt. I have been tempted to trde the tractor for a shft drive. I do some commercial snow blowing and some commercial mowing with this tractor. It only has 500 hrs on it so I didn't expect to be experiencing this problem. I also have been tempted to go to a larger pulley either on the electic clutch end or on the tensmission end just to see if that would cure the problem.
Have you tried adding a little fluid? I think you have to add it through the vent cap.
My owners manual says it is a sealed transmission and that I need to take it to my dealer. I'm not going to do that. It can't be anything that I can't repair without some advice. I will try to locate the vent and add some fluid.
# XXXXX is plug , with magnet under it. This should be port for any oil refill.
I will give that a try and will let you know if it worked or not. Thanks for your ideas. Maybe Simplicity is the answer.