Cycle it. Open and close the knife a few hundred times over the first week or two, with a drop of light oil at the pivot. Phosphor bronze washers in particular bed in with use. Do not force anything, and do not try to speed it up by loosening the pivot before the washers have settled.
New washers have not yet worn to match the surfaces they run against, and factory thread locker plus a conservative pivot setting adds resistance. Bronze washers especially bed in over the first weeks.
The detent also seats into its dimple over the first hundred or so openings, which is why a new flipper often fires better after a fortnight.
One drop of light oil at the pivot, then cycle the blade slowly and repeatedly while watching television or on a phone call. A few hundred cycles across a week does more than any single intervention.
Wipe away any oil that appears outside the pivot.
Do not force a stiff blade with a flick that risks your fingers. Do not loosen the pivot straight out of the box; you will end up with blade play once the washers do bed in.
If it is still stiff after a couple of weeks of use and oil, then adjust the pivot. See adjusting pivot tension.