Copy/paste from one Excel sheet to a sheet in another file pastes only values, not formulae

Hi, I'm embarrassed to have such a simple problem but be unable to solve it myself, but it would really help if somebody can help me.

I have Excel workbooks/worksheets to calculate stuff for invoicing and they are simple--no macros, no references to other worksheets, etc.  I wanted to copy/paste a newer, functioning, worksheet into another workbook, but it copies and pastes only values, not formulas.  Well, that's essentially useless.  I can't even copy a single cell and paste it with its formula, let alone a whole sheet.  I tried "paste special" and to select formulas off the pick list there, but the program won't allow me to pick any of those special paste options.  What do you think may be wrong?  Thanks very much in advance