You checked the deck belt, but did you check the motor drive belt. It's on the motor turning the front roller. Push on the belt, it should only deflect 1/2" or so.
Yes, I did. It is tight and not slipping. The motor just comes to a stopand hums; just as if it can't pull it. I know it will do that if you have the speed set too slow but I like to use it at about 3mph. Can I put some kind of lube under the big belt? I know we don't want that belt to get too oily or it will slip.
Yes, all you need to do is clean and lube the belt/deck surface. Here are the instructions and products here:
Lube & cleaners
World famous lube instructions