SS 15 Lacks Power

Hello Group,
I have an SS 15 and it worked fine last year and earlier this year, but now it is lacking power to move, engine is fine and checked belt and tightened, but the tractor moves very slowly, I have to use 3rd to get any where,
Any ideas?
Dan

Two major propulsion elements to look at here: Engine and Transmission.

Is the engine getting up to speed?

Belt slipping?

Are you sure the transmission isn't in low range? Can you shift it back and forth from low to high?

Many people here would be very glad to help, but we will need more information please. :-)