my groups (categories) won’t keep the names in, despite clicking save?

I have set up various groups/categories in Windows Live Mail for sending group e-mails and when I click on them, they appear to have everyone in, but if I try to send an e-mail, there are either no contacts in there (and I get the message, 'you must type a recipient in the To box') or there is only one recipient address in there. When I click on the category to edit it, the original contacts have all disappeared! I try putting them in again and click save and they don't stay there!  It used to work fine, what is happening??