Windows 10, 64bit system

Hi

For some reason I've lost the ability to normalise objects either individually or multiple selections. In brief, I select the first object I norm it through the norm dialogue, the object is normed. I select the next object and nothing happens. I try using the quick norm method, the object's audio is flattened. I'm pretty sure I accidently changed a setting somewhere, but I can't think what or where... Any help gratefully received.