We have a custom entity, Sales Rep, which is a field on the opportunity Form (also on Account, Contact, Etc.). For members of our sales force that are CRM users, they can't update the Sales Rep field on an Opportunity even when they are the owners. Same concept applies for Accounts, Contacts, etc.
I assumed this had something to do with Security Roles but I didn't see anything there that seemed to be the answer. Maybe it is an issue with Relationships?
As always, any help is appreciated,
Matthew Partin