Contacts keep disappearing from a newly created groups

I keep trying to add email addresses to newly created groups.  I am following all the instructions correctly - I've created and managed groups before in the past in an older version without any problem.   All the names get added fine, the group is saved.  When I go back in a second later, loads of the email addresses have gone.  More than half of them in some cases!!!! What is going wrong?  This was happening initially on my iPad so through it was due to being on a mobile device.  I've since tried on my Windows PC and exactly the same problem is occurring.  Please help as I am getting very cross!  Thanking you in advance.