Copying sheets sometimes impossible

[dan]

MS Excel 2003 (11.6113.5703) on Windows XP Professional SP1

I want to copy a sheet inside the same workbook by drag-and-dropping its tab
or by
using Move from the context menu (by right-clicking the tab). It usually
works fine but I have a file where the result is random: sometimes nothing
happens.

The similar situation when I drag a sheet from another file to that specific
file.

Thanks,
Dan