Insurers can no longer reject customers with expensive medical conditions thanks to the health care overhaul. But consumer advocates warn that companies are still using wiggle room to discourage the sickest — and costliest — patients from enrolling.

Some insurers are excluding well-known cancer centers from the list of providers they cover under a plan; requiring patients to make large, initial payments for HIV medications; or delaying participation in public insurance exchanges created by the overhaul.