Cells not updating/formulas not calculating

For some reason, the formulas in my spreadsheet aren't automatically calculating anything. If I change the data that the formulas use, the result stays the same until I click the formula button, 'f(x)', and walk through each step. For example:

I set cell A1 to 5
I set cell A2 to 6
I set cell B1 to "=A1+A2"
At this point, B1 = 11
I change cell A1 to 3 and cell A2 to 5
At this point, B1 still = 11
I hit 'f(x)' and hit ok
B1 changes to 8