Contacts disappearing from Groups

I am running Windows 8.1 on a Dell laptop.

I have a number of groups set up in the People page on Outlook.com.  They have existed for some time and I have used them repeatedly to send out broadcast emails.  Recently (following the 8.1 update?) I found that recipients had not seen the emails I had sent.  On checking I found that the groups were either completely empty or contained a handful of contacts where there had previously been a considerable number.   I have tried changing the group names and rebuilding them, but they become empty soon after.

This is a massive limitation on my use of Outlook.com.  Can anyone suggest what the problem is, or a workaround that will let me mail out to lists in one form or another.

Thanks