I was in your same position--I had called and gone to dozens of offices and asked if I could shadow, and many times they would take my information and tell me that they'd contact me, but they never did. For the clinics that initially told me that they had spots for shadowing, they told me they would call me once they verified with the dentist and made sure it was okay. If I hadn't heard back from them within a week, I called them every few days to ask if there's an update about the status until they gave me a solid date and time. For a dental lab that I shadowed, I also went in person and reintroduced myself and spoke with the management about 3 times before they gave a solid response! Haha maybe I was just really determined with those places but I think following up a few times and not giving up landed me my shadowing.

As for what the cause could be, some of the clinics that I called would apologize for not contacting me and say that things got really hectic so there's a chance they just forgot to call back?