All cadets are required by regulation to see the dentist at least annually for an examination and cleaning and filling any cavities. The cadet dental clinic, like all military clinics these days, does not have an overabundance of staff. These people are also human, and get sick, or get tasked to do something else outside their normal duties. If the dentist cancels an appointment, they are supposed to offer a reappointment within a reasonable time, and to my experience, they do. If the cadet cannot get an appointment within a reasonable time, each company has a cow or firstie in charge of the company's dental affairs, and using the chain of command will bring results.
Dental clinics have emergencies, too, and anyone can be bumped from an appointment.
Most often, however, it is cadet activities that cause missed appointments. One can make an appointment for a checkup, which will include exam and x-rays, and advice for further work, to be done in subsequent appointments, such as cleaning, cavity repair, extractions, etc. Should it happen that Keller Hospital does not have a dental hygienist for some reason, this service might be temporarily suspended, but it's likely the cadet would be so informed and advised to check back in three weeks or so.