Should be a simple fix.

I am summing numbers in a column. When I enter new data in the column, the
sum doesn't re-calculate automatically. If I re-type the same sum formula or
click on the sum cell and hit return again, it re-calculates. It used to do
this automatically with every new entry in the column. Have I unwittingly
changed a setting to make this happen? How do I reset it so the total is
automatically kept current?