Relative References Not Updating Properly

I have a worksheet with cells containing a simple formula like =b1/a2. If I copy that formula to a new column wanting the formula to become =c1/b2, what I end up with is =c1/a2. I can't figure out why the denominator won't change along with the numerator. If anyone can explain what's going on and if there is a way to make it work the way I think it ought to, I would be most thankful.

TIA for any suggestions.
pb