what I had though and that is the 3PK belt would have to go toward the motor on the crank exposing the 1 rib. I personally ran this configuration for years on my TT and STILL have this setup on my VerTT since I opted not to buy a new crank pulley.
I would start by checking tension or replace the belt all together with something like Gates. If I remember, I can check the belt length when I get home.