Heel Slip
Heel slip refers to vertical movement of the heel of the foot relative to the heel cup of the shoe. It can arise from several different things, and a small amount of heel slip is not a bad thing. In fact, it is generally desirable, especially on a new shoe, as if there is too much pressure forcing your heel into the heel of the shoe, any movement will result in rubbing that is much more painful than normal wear.
The most common thing people perceive as the cause of heel slip is a shoe that is too big. This is sometimes true, and if you know your full Brannock size and are sizing relative to it using reliable sizing advice from a source like this website, it can happen if the last has a much taller instep than your feet do. However, the overwhelming majority of people out there are wearing shoes that are too small, and for most people heel slip is a consequence of their shoes being too short.
When the shoe is too short for your heel-to-ball size and you have improper arch alignment between your feet and the shoe, the point where your foot wants to flex ends up ahead of where the shoe wants to flex. In this scenario, when you lift your heel to begin to take a step, your foot is trying to flex the shoe in a place where it is much, much more difficult to do so. As a result, what ends up bending the shoe is not the “pulling” motion from the flexing of your foot but the pushing of your instep against the instep of the shoe, and until the pressure on the instep of the shoe is so great that the shoe flexes to alleviate some of it, your heel moves independently of the heel cup of the shoe and you experience heel slip. If you size correctly relative to your heel-to-ball size, it takes much less force from your foot to bend the shoe, which in turn means less heel slip.
Beyond just sizing, there are other variables that can impact heel fit, such as the width of the heel of the last, the size and shape of the heel counter, as well as the pattern. If you are wearing the ideal size for your feet in that last, these are incompatibilites, meaning you either move on to other things that fit better, or accept some fit imperfections, depending on their severity. Sometimes, a tongue pad is enough to put your heel in the right place. Other times, things just don’t work out and it’s best to let go.