Most missed care is missed for reasons that have nothing to do with the illness.

Most regions have clinics that see people regardless of ability to pay, charging on a sliding scale based on income. They are consistently under-used, mainly because people assume they will be turned away or billed later.
Sliding-scale fees usually require proof of income, and the proof is simpler than people expect — recent pay records or a signed statement. Arriving without it often means paying the full rate for a visit that should have cost very little.
A clinic open only during working hours is closed to anyone paid hourly. Where evening or weekend sessions exist they are usually under-advertised, and worth asking about directly rather than looking for online.
Facilities receiving public funding are generally required to provide language assistance at no cost. Families routinely bring a relative to interpret instead, which is legal and frequently produces worse care, particularly for children asked to translate for a parent.
Instructions given verbally at the end of an appointment are poorly retained by anyone. Asking for the plan in writing, or writing it down before leaving, prevents a large share of the follow-up problems these clinics see.
A second person remembers what was said, asks the question the patient was too tired to ask, and makes it considerably less likely that an appointment ends with nobody understanding the plan. Clinics are used to this and it slows nothing down.