You aren't necessarily doing anything wrong. Some people just have very stable temps! But you might want to make sure your thermometer batteries aren't dying.
As for charting online, I think most of us here use fertilityfriend.com. It's easy once you get the hang of it! Once you're signed up and have got a chart going, look at the menus on the left side of your screen.. go to Sharing and then Home Page Set Up and it will give you the URL that you can post here.