Battery doesn't seem to want to charge?

Several weeks ago, I accidentally left the lights on and my wife went to go to her mothers and noticed. She tried to start the car and it was sluggish but started after about 5 seconds or so. It has been maybe 2-3 weeks since this incident and the car still starts sluggish but starts. I have driven it several times for several miles over the last few weeks and I figured by now the battery, being not even 6 months old, should have recovered but this has not happened. Finally today I went to wal-mart for some tools and left the store after about 10 minutes. I tried to start the car and it struggled and struggled and never did start, when I did try to start the lights dimmed and the door gong that lets me know the door is opened started sounding like it was dying but when I let up from trying to start the lights lit up and the sound returned to normal. I had to get a box charger to get the car started to get home.

Any ideas why the battery has not recovered?