Leads were sent the email and clicked on a web page link on the email. However, the web page activities were not associated to these leads.
Check that the link clicked in the email has tracked link enabled
If the web page activities are associated to other records (e.g. duplicates), then:
Check who was 'cookied' on the web browser before the visit web page activity was recorded for the other records. This can be checked by looking at the web page activities recorded in the activity log.