unprotect VBA error
 

I am getting an error message when I try to use VBA code
to unprotect a worksheet. The code is simple:

Sheets("sheetname").UnProtect

There is no password assigned. But I get an
error: "Unprotect method of worksheet class failed."

What am I doing wrong? Any ideas? I have no problem
protecting a worksheet with VBA code (Sheets
("sheetname").Protect
DrawingObjects:="True"... etc...)

I get the error message only SOME of the time, which
confuses me the most. Any help is greatly appreciated.
Dan