What exactly does your SUM function look like?
What if you try to manually add two cells in your sum range, does that work?
i.e. = A1+B1
Also, if you use the ISNUMBER function on one of these cells, what does it return?
=ISNUMBER(A1)
Note that just because the cells are formatted a numbers does not necessarily mean that your entries are entered as numbers. For example, if you type in a single quote before a number, it will enter it as Text. Usually, a good indicator of this is if your entries are left justified instead of right justified, they are probably entered as Text.
If you highlight a column, and select Text to Columns from the Data menu, and then click Finish, this will change all those entries from Text to Numbers.