I set up values in a number of extra fields using a variety of types of value - dropdown, multi-list, text fields, etc. Initially this worked without any problem and the extra fields appeared in each Item.

Now I find that when I try to set up a new extra field there are no values attached and when I edit the existing extra fields the values have completely disappeared.

do you have the latest version of k2? If yes, then i must inform you that i have noticed a bug with the aliases of the extra fields. When you firstly create an extra field it autocompletes an auto alias from the title of the extra field. when you change the alias as you wish and save it everything is ok, but if you want to edit that extra field and save it ,although you havent changed your alias, you will notice that you alias has been changed. so if you try to get the items values you simply cant because the alias has been changed