Excel Sum do not add automatically

[Bobbi]

i have just installed a new Office 2013 recently and i encountered this problem in excel:
when i put a Sum formula at the end of a column, it will add up all the values in that column, but when i add in a new value, or change a value, the sum at the end column do not change automatically ie it does not add up to the new total again. why? it does not happen when i used the old system last time.