These three reasons could explain the sales rep's experience of being able to edit some opportunities associated with accounts they own, but not others. If the organization-wide defaults for opportunities are set to Private, then the sales rep can only edit the opportunities that they own or that are explicitly shared with them. If all provisioned Opportunity object permissions enable Read access with all accounts the sales rep owns, then the sales rep can view but not edit any opportunity associated with their accounts, regardless of who owns the opportunity. If some opportunities associated with the sales rep's account are owned by other users, then the sales rep can only edit those opportunities if they have been shared with them or if they have a higher role in the role hierarchy than the owner.