Answer
When no resources show up with Apply Constraints enabled, check these candidates in order: whether the Work Task's Company differs from the resources' company; whether the relevant resources are actually flagged "Used in Service"; whether the Work Task's Service Organization differs from the resources' own Service Organization; and whether the specific resource group referenced by the resource demand genuinely has any resources in it — any one of these mismatches will silently filter out otherwise-valid persons once constraints are applied, producing exactly this symptom.