Whenever I dispense orthotics for the first time, I always re-lace a person’s tennis shoes. Without orthotics, your arch height is flatter than it is with orthotics.
By increasing the support undeneath your arch with an orthotic, you’re effectively raising your arch height, which in turn lowers the front part of your foot. If your shoe lacing doesn’t take into account this new foot shape, then you foot will act like a teeter-totter, popping your heel in and out of the shoe with each step.
Also, the best way to ensure proper fit is to completely cinch snug the laces closest to your toes and not the laces securing your foot to ankle. The more form fitting the shoe, the better the fit.