Click the SUM cell and verify that the formula refers to the rows you intended to add. Then try =ISNUMBER(B2) in an empty cell, replacing B2 with an amount that appears to be missing from the total. If it returns FALSE for a cell that looks like an amount, the value may be text that SUM skips.
01Rule out a zero that is actually correct
Section linkInspect the formula bar before changing the data. A formula can point to an empty range, the wrong column, or an older range that ends above newer rows. If the range is right and the entries are numeric, check whether positive and negative amounts cancel each other out. A zero total can be correct.
02Convert a sample beside the original
Section linkFor a confirmed text amount in B2, enter =VALUE(B2) in a spare column. Compare the result with the source before copying the formula down for other amounts. Total the converted cells and compare a few rows with the original amounts. Keep identifiers such as order numbers in their original text form.
03Check the import format when conversion fails
Section linkIf VALUE returns an error or an unexpected number, check for currency text, decimal separators, or values that Sheets reads as dates. Check the spreadsheet’s locale under File → Settings before cleaning a copy of the imported column. A locale change affects the entire spreadsheet, so don’t change it just to repair a shared total.