Are you doing anything else differently? Changing your diet?
20 min 5 days a week is considered a bare minimum for good health maintenance/circulation.
What sort of exercise are you doing? If it's walking you definitely need more, but if it's some type of very intense workout then 20 minutes may be fine. You can try it for a few weeks but if you don't think you are getting any results, then you should probably increase your exercise sessions to 45 min - 1 hour each.