Until there is battery capacity spare Regen will not cut in. Obvious really as where would it store it? But it took abit of getting used to as my house it on top of a hill and I use that regen to slow down but not when fully charged.
97% sounds about right for a used Twizy. Mine is over 4.5 years only 12,800 miles but reads 97% when reconnected. However I have the older charger that runs on for hours after 100%.
The other key point is, I have found it is possible to run the Twizy without full regen! For full regen turn the Twizy key all the way and wait for the SERV light to go out before letting go of the key. If I do not wait (about 10secs) then I do not appear to get the full regen. It still displays on the dash but doesn't slow car or put charge back into battery.