What the heck?

Why are the lights on the bottom portion of my pre-lit Christmas tree not working?? This is the second area of the tree that has gone out on the "if one light goes out the tree stays lit" Christmas tree since Sunday! WTF? We've only had this tree for five Christmases, and we paid MUCH more than we should have for it. I didn't expect to have to re-light a freaking pre-lit Christmas tree!!! Man that's irritating.