It is likely that the customers have reached their purchase limits for that product. Probably a misconfiguration in the settings. Please can you describe, what you want to achieve? i.e. Are you trying to set a limit of 1 per customer for the introductory box? For what period of time?
Please can you let me know the settings you have under:
-General > Mode -General > Date range -General > Order statuses -General > Guests -General > Block guests from buying products > Block method -Limits tab and Formula tab