Option Return will make a carriage return in a cell. A carriage return is not a number, which makes the value in the cell not a number. I believe the only way this can even happen is if the cell is formatted as text. This can cause problems
One way to tell if a cell contains an actual number vs a string of numeric digits is to look at which side of the cell the value aligns with. If it naturally aligns to the right, it is a number. If it aligns to the left, it is text/string. If you have manually overridden the text justification in the cell, both will align whichever way you told it to do. You'll have to change it back to the justification that looks like an "A" surrounded by text to see any differences.
"Numbers" that are actually string values will be ignored by the math functions (SUM, AVERAGE, and many others). The +-*/ operators will work with them but the math functions will ignore them.
If the cells contain data (not formulas), and they look like numbers but they align to the left, they are most likely formatted as text. Change the format to "number" or "automatic". Any that don't move to the right are still text and you'll have to figure out why. If you had manually set the text justification, you'll have to change it back to see any change.
If the cells contain formulas, you will have to modify the formulas so they result in actual numbers, not strings. I've seen formulas like IF(A1="yes","1","0") instead of IF(A1="yes",1,0). The first results in text, the second results in actual numbers.
