Why do I only get 5 clear channels on my Panasonic plasma TV with an indoor antenna?

I recently hooked up an indoor antenna to my Panasonic plasma tv. I bought the antenna from best buy, was told I should be able to get clear channels but I only get channels 2, 7, 9, 11, and 13, anything higher is broken or frozen. I tried moving the antenna and rescanning channels on my TV. Do I need a converter box or will this be a waster as well? Any information will be greatly appreciated.