Express
A few ideas which may help
1. Check your data, are there numbers which have been imported as text? Are there text values with extra leading, trailing or internal spaces ? Is the case of your source data consistant eg DT001 may be evaluated differently to Dt001
2. Have you used Relative & Absolute references correctly?
3. Are you using v/hlookups on unsorted data?
4. Go to an erroneous cell and use the auditing tool to trace precedents, you can keep clicking on Trace Precedents to go back several levels, make sure your logic is correct
5. With complex formulas, start in the inner level and check each layer as it is built up
6. have you used any Array formulas, check they have been entered correctly with Ctrl Enter
7. Are there anyy patterns to the errors, every 2nd row etc
Good luck