This one comes up all the time, and I thought it was worth a post. You enter a formula and it calculates correctly. Then you fill or copy the formula and you get the same answer, but you know the answer should be different.
Nine times out of 10, the problem is that calculation is set to manual. Take a look at the lower left of your spreadsheet.
If it says “Calculate”, then calculation is set to manual and it needs to be automatic if you expect the formulas to change. Go to Tools > Options > Calculation and select the Automatic option button.