^^^^
Actually, heat source does have somethin' to do with it maybe. Induction is heating very evenly, so you won't get circulation until there's a strong upwelling all over, and the walls of the vessel force some of the wort to go somewhere (i.e., to the center and back down. Or the other way I suppose.) OTOH, I use direct flame. I heat my kettle asymmetrically. (It sits over two burners on my stove, so it's easy to set them differently.) This naturally causes the wort to rise vigorously on one side and descend on the other. I get a good roll at a fairly minimal application of flame. [Commercial kettles that heat too evenly to induce circulation at minimal heat incorporate mechanical agitation.] I wonder if you could find a way to mimic what I do, inducing a more vigorous boil on one side. I don't fully understand the operation of induction cookers (the range over which the field is effective,) but -- could you place some random stainless hardware on the bottom of your kettle on one side? Would that generate more heat there? Just thinking.