You might notice that some constituents do not have the communications tab updated shortly after they are sent an email in BBIS.
In BBIS, go to Administration -> Sites & Settings -> Schedules and take a look at the Blackbaud CRM Integration settings. Constituent records are updated based on those settings. The more frequent the updates, the quicker the constituent records will be updated to reflect that they received the communication.
For additional information about how records in BBCRM are updated from BBIS email activity, please review Knowledgebase article 106588