Cant successfully add users shared inbox

[JTracey]

User A grants 'Reviewer' permissions for their inbox to User B in OWA. When User B is in OWA they right click their name above 'Inbox', select 'Add shared folder...', enter in User A's name, selects User A, and selects 'Add'. User A's name appears in the left pane but doesnt list anything under it after expanding the folder. 

When User A does the same process to User C it works as it should. User C can see all contents of User A's inbox including sub-folders. 

Using PowerShell I tried the following which gave the same results as paragraph 1:

Add-MailboxFolderPermission -Identity User A@domain.com:\inbox -user User B@domain.com -AccessRights Reviewer

What could be causing this issue between User A and B, and how can I solve this? 

[#2]

I have seen this sometimes and IMHO is a kind of delay in permissions propagation that not always happens

[#3]

When you grant folder-level permissions, you need to make sure to grant permissions on the Root/Top of information store folder as well. As well as to any other folder in the "path" to the folder you want to share.