Which belt, and what brand do you have on each path?
I had to buy Gates Micro-V belts because my local supplier was out of Gatorbacks for my accessory belts on both L67'* last week. I was able to get Gatorbacks for both SC belts.
The Micro-V is a tighter fit because the grooves are tighter than the width of the ribs on the pulleys. I believe this is by design to 'pinch' the ridges, but it makes it tough to put them on, effectively making your belt path longer if you get what I mean.
Both my accessory belts are making a little noise, but I assume they'll 'break in' at some point. I'm not pleased, and will probably be replacing the Gates at least on my SSEi in the future with a Gatorback simply because of the much smaller than OEM pulley on my 160amp alternator. The Gatorback will turn a smaller pulley much better.