I have two users, A and B, in the same user group (neither are in other user groups) but only one A can get access to the list.
The list has permissions based on user groups (i.e. no-one has been given access directly). There are no item level permissions.
I used Check Permissions and this confirmed A had access because of being in the user group and B did not have access.
I removed B from the user group and re-added him to the group. Now both users can access the list.
Why would user B not be able to access the list in the first place?