I am unable to refresh any of my sandboxes We have 20 remaining Developer sandboxes We have used 5 Developer, 1 Partial and 1 FullCopy.
It was because we recently had removed our license for the FullCopy sandbox... meaning:
Even though the FullCopy was locked we were still using 1 Full Copy more than were were provisioned.
And hence Salesforce prevents refresh on all sandboxes until we are within sandbox limits.
Solution: I deleted the FullCopy and the refresh buttons returned.
NB: if you delete it, it is gone gone.
It was not related to the "Manage Sandbox" permission I already had that set as system admin.