If you're scheduled to be working/volunteering at the hospital, you should be doing that. Shadowing is just following a doctor around and not really doing anything, so I can see how a scheduler/manager wouldn't like you doing this. If you're looking for shadowing opportunities, try inquiring at a teaching hospital.
Try to build a rapport with some of the physicians you see when volunteering. You could parlay that into some shadowing opportunities. The hospital I volunteer in allows a single shadowing shift for volunteers so I'm having to do some legwork.
Being super-friendly with the nurses and techs can be beneficial too. They can introduce you to physicians and recommend doctors who would be receptive to shadowing. Heck, just being super-friendly, hardworking and humble all the time has its benefits. I've been fortunate enough to be allowed stand in on a variety of procedures performed by doctors/NPs/PAs ; intubations, trauma intake, cardiac arrest. I've even helped to clean and suture quite a few wounds too. Most of the other volunteers spend their entire shift standing around, stocking carts and blanket warmers.
As others have said, ask the physicians themselves instead of the hospital, and try to schedule your shadowing for times when you are not supposed to be volunteering. The hospital probably won't be a huge fan of you shadowing instead of volunteering during hours you are supposed to be volunteering, but they probably won't care if you come in at some other time and shadow, as long as the physician agrees to let you shadow him/her.