What is the make and model of your laptop? Many of them now days have settings in programs that are 'battery calibrators' or 'performance enhancers' where the battery will not fully charge if plugged in and powered on. This is supposed to extend the life of the battery within these settings.
For example, earlier this year I had an MSI Stealth laptop (can't remember the model) and I had this issue because of the MSI software on it had a setting that would the battery at a certain level while plugged in and turned on. This was to extend the overall life span of the battery. I had other issues with the laptop, so I returned it. I am now using an ASUS ROG Strix II laoptop and there is software on it called 'My Asus'. In that software there is a setting called 'Charging mode'. The options are 'Full Capacity', Balanced mode, and 'Maximum lifespan' mode. In max lifespan mode, the battery will only charge to 60% and in balanced mode, it will only charge to 80%. Full capacity mode is the only mode it will go to 100% with.