It isn't "the ammo". It's the bullet weight.
Even some 1:9" twist .243s have trouble stabilizing long bullets like the 100 gr Core-Lokt. Don't blame the brand or ammunition, other than the bullet in that particular load.
Pick up a box of the Remington 80 gr SPs or HPs if you can get them in your area. I'd be willing to bet they'll group just as well as, or better than, any other non-premium factory ammo.
I am absolutely NOT a fan of "economy"-grade ammo.... with the exception of Remington Core-Lokts. The . stuff . just . shoots in nearly any rifle.
The rest of it (Winchester Power Points, Federal Game-Shock / Fusion, etc.) is a waste of money.
Also keep in mind that it's a brand new rifle. As barrels break in and actions settle into their stocks, performance and accuracy will change. (Velocity, point of impact, and group size.)
It's quite possible that the rifle is running "slow" right now, with lower than expected velocities. As it breaks in, the velocities may come up enough to allow long (heavy) bullets to stabilize better.
So, you may be able to use those extra boxes, down the road.
For the time being, avoid 100+ gr bullets and long, slender 95 gr bullets.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."