Look at the spindle pulley. Some of these pulleys can have the centers break out of them.Your model # from below the seat would be helpful if you need further help. Pitch could be an issue - though usually these don't have individual front adjusters.

P/N is 13AX60TG766. It's a 2006 model
Thanks

If the pulley / spindle is happy, pull the deck and look at the rear tensioner. The bushing gets seized in the arm from time to time. Temporarily loosening the bolt a tad will get you by until the parts come if this is the case. (Provided the locking nut does it's job.)It doesn't move much- but if it doesn't move at all it will affect belt tension under load.

I'll check that out in the morning. I am curious why the tensioner would only cause a bad condition on one side and not both. Maybe I'll understand more when I get the deck off and see it.
Thank you for the suggestions. I'll report back tomorrow.
Bill.