Multiply two cells and recalculate if a value is changed?

Ok, absolute newbie to spreadsheets here. I have entered the function (eg: =c1*d1 to display result in e1)which works but if I change a value in one of those C1 or D1 cells, it doesn’t update the result cell. Any help much appreciated.

Have you Menu/Tools/Cell contents/Autocalculate on?