My Sigma DTS 1106 bike computer isn't calculating...

Question

Since the purchase of my computer last summer, it has only calculated 31 miles for me when I have put on several hundred. I changed the battery in the component that attaches to the fork thinking that might be the problem, but it still isn't opporating. The screen is functioning, just not registering the miles. I can't seem to locate the owner's manual either in my garage or on-line. Help!
Ellen