If you are copying it into another sheet and you don't put
it in the same cell, you may be losing the range. =sum
(a1:a20) in first spreadsheet will not work in next one if
you put the formula in a10 since you lost the range. Or
you could be using a range name in the first sheet that
does not exist in the next? =sum(myrange) where myrange
is in tab 1 but not tab 2.
if you want the result of the formula to appear in your
new sheet, why not reference back to it (so that it will
change dynamically in both sheets when it changes in the
first) tab 2, cell A20 "+tab1!A15"
If you really, really want to copy the formula exactly,
then go to the original cell, hit F2, home, ', enter.
Then copy that to the new tab and then go back and delete
the ' (which signifies text).
HTH.