A. Wrong belt installed. (unlikely)
B. Most likely, the belt routing is incorrect. Check the diagram up front for the proper routing of the belt. The tensioner has plenty of adjustment on it. It fact, with a normal handle length of a ½” drive wrench, you still need a pipe extension to pull that baby back all the way, there is that much pressure against the spring. It is very easy to miss one spot in the routing and still have the belt stay on with the available travel on the tensioner. I would recommend another set of hands to assist.
2007 LBZ 3500 LT3 DRW Crew Cab, Sulastics, Putnam XDR 15K, B&W 30K-Turnover GN+Companion, Ride-Rite Air...EFI'd by Rob
2008 LMM 3500WT DRW Crew Cab, 8 'Flat/GN, Workforce Alum Toolbox, Ride-Rite Air Use-ta Haves 2006-6.0 3500 WT DRW ExCab Flatbed, 2005-LLY 3500 LS DRW Crew Cab, 2002-LB7 3500 LS DRW Crew Cab I Live my life in such a way that when my feet hit the floor in the morning...Satan shudders & says "Oh shit, he's awake!"