on the back of all cassette tapes there are 2 plastic tabs, designed to be 'snapped' to prevent overwriting of the recorded material, check the tape and ensure the tabs are in place. if not put a piece of tape over the hole and it'll work. and yes, there is a small arm/pin which contacts this tab and if its missing the arm goes in and you cant press the record button.
kevin-g, November 2010