limiting a combo box in a subform

I’m needing to limit a combo box in a subform by a value in that subform. Having no success. The Combo is populated by my table “Supervisors” which has two fields - [ClientID] and [SupervisorNames]. (A client has multiple supervisors.) The subform includes a [ClientID] and contains the combo box to select the appropriate Supervisor. (There will be several records in this subform for each item in the Parent form, each with a different Supervisor). I need to limit to Combo to only the Supervisors associated with that [ClientID]. I’ve tried several criteria statements in the query, but to no avail. Any assistance is appreciated!!!