Folder Writer/Editor cannot edit pages in a folder

Help! One of my wiki users is losing access.

I have a number of folders. Some folders are open to all users, some folders are only accessible to specific users. One user ("User A") was assigned as a Writer to Folder X. For the last week, User A can seems to have Read-only access -- User A can look at pages in Folder X, but when she tries to edit, she is told "request access".

From my end, her access seems to be set correctly. I have tried removing and restoring User A's access to the wiki and to Folder X, but she still can't edit anything in Folder X.

Any thoughts? User A is the only one reporting this problem, but several others may not have looked at "their" folder recently.