Are you selecting one or more paragraphs? Some testing indicates that you
should *not.* Instead, just place the insertion point in the paragraph whose
numbering you want to modify. Then right-click to see the relevant options.
To make an item part of the immediately preceding list, use Continue
Numbering. And to create a new list, use Restart at 1. You may have to set
these options repeatedly, for several list items, if those items have been
"disconnected" from the list.

Indeed, manual restarts may require a lot of work!  :-(

I'm sure you'll find it easier to define a numbered style and restart it
using LISTNUM fields or define a multilevel list where you can use higher
levels to restart lower ones. See
http://word.mvps.org/FAQs/Numbering/ListRestartMethods.htm.