How old is it? After a while batteries just lose their ability to hold a charge after a while and it's not uncommon to see batteries with 3/4 of their original life after a year or so. My laptop is 8 months old and it's telling me that it charges to 83%. If you want a battery that will hold a full charge you're going to have to buy a new one.
Sorry for posting so late but I just ran across your post. Perhaps my answer will help others should they run across this.
A number of laptop manufacturers include a manual setting either via software or via BIOS that caps the charging of a laptop battery at 80 percent. This is offered because only charging to 80% will increase the service life of the battery. The trade off is run-time.
A lot of times this feature goes by "Optimize for Battery Cycle Life" or something similar to that.