Anxiety is the response to fears. Medications help symptoms but not the underlying fears. OCD behaviors are about fear too. The often unconscious reason for the ritual is that itl will prevent something fearful, like an exaggerated type of repelling bad luck by the 'knock on wood' ritual.

I hope you will find another child psychologist. Play therapy is very useful not only in what it reveals but in helping a child open up to the therapist. Part therapeutic and part ice-breaker; very important and helpful to the therapist.