Anyone had a problem with users not being able to login ?

I have one user who can login to the client but not WA, they put in the username and password then it looks like its going to show their mailbox but then returns to the login box. I have tried the account on different PC's and browsers, the same problem exists. Other users of the PO are fine, I have ran a few gwchecks too of the mailbox.

Any ideas ?