Craftsman rider mower GT3000 w/20hp Kohler engine is running slow up hill?

My rider is starting to have a hard time making it up hills in my backyard. It use to have no problem but in the last couple of years it seems to have a harder time. Any idea as to what I should check or fix? Thanks all!|||The most common problem is the drive belt being worn and slipping. Does the engine rpm stay the same and the tractor slows down or does the rpm drop like the engine is lacking power? If the engine is slowing down and lacking power I would say that it needs a tune up and maybe the carburetor cleaned. That would be air and fuel filters, spark plugs, valve adjustment, oil and filter change. I would first look at the drive belt. Also if it is an automatic transmission there is a purging process in the owner's manual.|||Don't know which mower you have but could be a loose ,stretched belt or if it has a rubber wheel disc drive the wheel has to be replaced. Looks like a record player if wheel in center most position fast, outside position slow. They wear down in size. Hope this helps