Are you lubing up the tip/head? That’s a recommended first tip, if you’re just not getting a good seal. Did it come with more than one head size to try also?
Some people also do have a preference if placement, whether the clit is dead center or to one side or another. And my wife’s preference is for giving it some movement, like holding down for a tight seal and then rubbing circularly while it throbs.
And it may just be a preference thing, too!