Final answer:
The contacts are not receiving the welcome emails because they fell below the High Water Mark, their account has deliverability issues, or the entry event is triggered too often.
Step-by-step explanation:
The reason that some contacts inside the data extension updated their preferences after the launch of the campaign are not receiving the welcome emails is likely due to the fact that the contact fell below the High Water Mark. The High Water Mark is a system-level setting within Journey Builder that ensures contacts do not re-enter the journey if they have already completed it. If a contact updates their preferences after completing the journey, they will not receive the welcome emails again because they have already reached the High Water Mark.
Another possibility is that their account has deliverability issues. If their email account has deliverability issues, such as being flagged as spam or blocked by their email provider, the welcome emails may not be successfully delivered.
It is also important to note that the entry event may be triggered too often. If the entry event is set to trigger every day, contacts who have already completed the journey will continue to be evaluated and prevented from entering again. This could result in a reduced email volume and contacts not receiving the welcome emails.