They don't slip, they move with the foot, and don't make a slap sound. I have a perfectly flat sandal with two cross straps, but they fit. I slide my foot in, then have to arrange the upper strap slightly against the foot. At that point the shoe stays put. I walk, sit, cross legs with a foot dangling, they don't come off. I don't know where these responses are coming from.

I haven't had a problem with them but my foot is wide so are usually pretty snuggly inside the shoe so no moving around happening.

Yes! They slip and make a flip flop sound. The trick is to get them with a slight wedge to the sole so the angle keeps them on.