Outlook Client Tracking Issue

God day.

I have a problem at one of my clients with outlook client. There are 3 users.

This is the scenario:

User A send an email and CC user B and C. After the email has been sent, user A tracks the email. In user B and C's outlook it doesnt show as tracked, even after 24 hours, but it does show up in CRM under activities.

If user A does the same as above but track the email before sending it works fine.

If user B does the same as above it also works.

They are using Dynamics CRM 2013

What can the cause be?