It's far cheaper to buy a few different kinds of ammo. My guess (based on previous comments by others) is that Weatherby will ask what ammo you tried and then suggest others. You might as well try a few others first before going this route.
FYI, one bad ammo does not an inaccurate rifle make.
Also, don't be afraid to check the torque on the action screws, scope rings, etc to be sure they're tightened to specifications.
Just curious if you used a chrono on the weatherby? I used a accumark .257wby with 2 boxes of factory 110 accubonds. We got 3moa at 100 yards and as much as 175fps differance between shots on the same box of ammo. This left us scratching our heads. At $70+ a box I would have thought it would be better.