You can crank down on them until you're blue in the face, but what a strap system depends on for keeping your foot from going out the back is an old style slotted cleat, which I'm guessing whatever shoes you're running don't have. Using these setups with street shoes is always a little kludgy; people seem to expect that just because stuff is expensive it will work well, but this simply isn't what it's designed for. Some shoes have softer soles and enough tread that it's not an issue, but without a cleat you're always going to be able to pull out the back if you really try. The solution, and the reason so many people do get away with running them with sneakers, is just to pedal a rounder stroke so your foot stays with the pedal.