Why is my battery not charging when connected to power source?

Why is battery not charging when connected to the power source. There is charge going to my computer, (lightning symbol on battery), but no light on charger (green or orange). Battery life says normal. Message on battery symbol says 'Battery is not Charging'. It's been happening for a coupe of weeks and occasionally charges to 100%, but I'm not sure what triggers that. Thanks for any help.