1. Where's the discomfort?
2. What type of foot do you have, e.g. high instep.flat or flatter feet, narrow or wade? If you are sliding around because the upper is too wide try wearing socks (I
know 
) or wear a thin foot-bandage (non-compression) rolled to invisibility. Weirdly, it's better to have these types of backless shoes slightly tighter at first because the effort to make them (sometimes ballet flats are similar) stay on is agony for the whole leg.
I used to work in a (ballet and theatrical) shoemakers. RTW shoes are never going to be perfect, and the less adjustable and less material they have to secure the foot the less comfortable they will be to walk in because they offer no support. Slides, flip-flops, mules and non-adjustable sandals offer very little support so the wearer holds the shoe with the toes (ache) gets blisters as the foot moves or has to shuffle just to hold them on.
Think about fit for purpose. Fashion is one thing, but these were not designed as walking shoes. Slip-on mules (slides) generically, they were designed for shuffling around the pool or house as slippers, that's what they were designed for.
It may also be that flat or flat-forms don't suit your feet. Slides are actually better for a flat/flatter and wider foot because most of the owners of feet like that walk flat-footed/shuffle (i.e. don't roll through their feet) and take up all the space of the upper so it stays secure.