I have a sync set up to update various fields in my customer list in campaigns but one of my fields isn't updating.

We changed a date in CRM 2 days ago and the accounts are supposed to sync every evening but it is still showing the old date in Campaigns.  

I've checked the mapped fields and the field is now missing from the list. I tried adding it again but I get a message to say that the field is already mapped and it will not allow me to re-add it.