Cannot introduce function referencing to cells in different sheets

[Stevie]

Hi all,

I have a problem that is really driving me crazy.
I have a sheet in which I cannot enter a Formula that sums the values
form cells that belong to different sheets.

The cells were protected and locked, and the sheets were grouped.
I tried suppressing all of those features to no avail.

Could someone please give me a clue as to what I'm missing or what
else I could try?

Thanks in advance.

Stevie.

FWIW The environment is win nt 4.0 and Office 2000.