My child had learned how to use the potty but now seems to have forgotten how. What causes this and what should I do about it?

When a child loses a skill such as being potty-trained, I look for an emotional or physical stressor as these can trigger anxiety or aggression that make it hard for the child to remember how to use the bathroom. Many stressors are quite obvious, but low-grade sinus infections or unrecognized dental problems are very common causes.