I'm thinking the new design of this old favorite--they added an adjustable back strap--is what's giving me problems.  It cannot be adjusted to fit snuggly and my heel doesn't feel secure.  I may have to take them to the shoe repair man to see if he can do something to shorten the strap or lengthen the Velcro strip to allow me to tighten up the looseness.  Nice shoe.  I've had several pairs of the earlier version and was somewhat disappointed with the fit of this new one.