1. First, check the number format of the cell.
2. If the cell is set to show numbers as text, change it to the correct number format.
3. If the cell is set to the correct number format, then check the number format of other cells in the associated column or row.
4. If the issue persists, verify the Regional and Language Options settings for decimal and digit grouping symbols.
5. Lastly, check the conditional formatting rules to see if any of them are modifying the actual number format.