First, look for the obvious. Take the action from the stock, look to ensure there is clearance for all the moving parts in the new stock.
Check to see if the trigger pulls the sear down without resistance. Both with, and without the bolt in the gun. Check to see that the trigger, sear and spring are assembled properly. The spring might be cocked, or broken, preventing free and easy sear travel.
Does the rifle still have its two stage military trigger pull? IF not, then most likely someone has done trigger work, and may have gone a bit too far.
All else being equal (and it almost never is) bigger bullets tend to work better.