Thanks for all the recommendations/advice. I have a new long blade belt for the center/left blades and I'll get it installed this morning to see if that helps with the uncut grass striping on the left side. I don't have a short blade belt for the right blade, so I'll get one ordered and replace it soon. I'll also measure my blades to make sure they are 20.5".
Add a spring to the springs (#3) to add tension to the left belt around the pulley because it is longer than the distance around the shorter side of the right pulley.These springs can get weak over time,and that will make the left side slip easier than the right side.This is why the factory installed multiple springs on the idler on the left side blade.
Do this before installing new belt,you may not need a new belt after all,just save it for later.
Take off one spring and take it to a hardware store for comparison.A same diameter,shorter spring will be good,as long at the wire of the spring is the same gauge diameter is the same.You should be able to tell the difference by pulling on them to compare.All new springs of the stronger type will be better.Stronger is better,up to a point.Excessively strong springs will shorten the idler bearing life.
After years of slipping,the left side pulley may be worn to the point where it is not gripping the belt properly.The pulley pulls on the SIDES of the belt,not the center.That is why the profile of the belt is tapered,like driving a wedge into a block of wood.
The belt should
not go so deep that it hits the bottom of the pulley.The shiny side of the pulley should look to be the same width as all the other pulleys
If this is the case,which I suspect,change the pulley and the belt(after adding springs to idler).
Put a little paint or lipstick on the pulley shiny inside and compare to the right side pulley to see if the groove in the pulleys is the same width.