Why won't SUMIF work?

I have a SUMIF function in a worksheet that won't work. All the references are right but it treats the sum_range as if they weren't numbers. The weird thing is, if I edit one of the sum_range cells in any way it then recognizes it as a number and sums it. But it still won't sum the other cells unless I at least double click in the cell and then click away. I don't know how the entries I am trying to sum got in the cells. I think they were pasted or imported from a database. I tried copying and pasting into another workbook. When it was first pasted the sum_range numbers were aligned on the left. If I double clicked on a sum_range cell to edit it and then clicked away, or if I just manually entered a new number, the number was then centered. I think it is the formatting of the imported cells messing me up. Please help!

Brad