"Hollow" areas are the kind of defect you don't always see during setting, but you'll hear about later. A floor can look flat and still sound drummy after cure, especially on large-format runs. Usually, the cause is uneven mortar collapse, skinned thinset, or slight movement while the bed is still plastic. Catching it early is far cheaper than chasing it after closeout. In this article, we will discuss why voids form and how to keep coverage more uniform.