If one of your contacts unsubscribes from a particular marketing email type, they will not be marked as ineligible for your other email types. The action taken by the contact is only applicable to the specific email type they opted out of. Please be aware the contact will still appear as ineligible for email in the contacts tool until the next time HubSpot is able to successfully send an email message.
In the case you have contacts who are subscribed to all of your email types and are still listed as ineligible, they may have hard bounced. Take a look at this article about removing a hard bounce for your contact so you can begin sending email to them again.