As you walk around in your cycling shoes, your soles will wear down, and there will be increasing play/looseness in the connection between pedal and cycling shoe.
I've tried different glues and epoxies, but the best solution I have found to adding some thickness to the sole, is a glue gun. It's very quick to apply the glue, and you can use it almost immediately after it cools. It's easy to apply the correct thickness of glue. It is elastic so it moves with the sole, and if you put on too thick a layer, you can still engage the cleat because you can push hard to squish the glue.