The most obvious thing is misalignment, -the stanchions may not lined up parallel.
I would loosen off all the triple clamp bolts top nut and the wheel spindle.
Back off any mudguard stay bolts a bit.
Then, tighten the spindle, (hold on the front brake as you do this), next tighten the lower triple bolts
Next bounce the forks up and down a bit, and then tighten the top and the stays.
If the bars are still off centre you could try loosening the top triple, hold the wheel between your knees,
and see if you can pull/push the bars into line.
Anyway that's how I'd do it.