If your Customer is having issues trying to log in, try checking that the email address they are trying to log in with is the same as the Contact email address which is shown in the Back Office.
For each Customer, you must add the individuals who would Order within that organization as Contacts. It is the Contact email address which they will use to login.
If the Contact email address is the same as the address they are using, they should use the 'reset password' function and try to log in once more.