Every year around May time, campaigns do not sync across the integration as they should. If all normal checks have been done and the integration is still not syncing then it is possible your timezones are not set correctly.
This issue is easily fixed and is simply caused by your Dynamics Integration using the wrong timezone.
Select 'Settings' (the cog icon), then click on 'Options'.
Ensure that your timezone is set to GMT +00:00 Dublin, Edinburgh, Lisbon, London. Then re-create your Campaign Activity.
If it is already set to this then you can rule out the timezone as being an issue.