Perhaps you put it in wrong, twist it in with the switch bit fully extended (looks too long, but will adjust to fit when you place in)
in fact:
Brake pedal - Assembly overview
Adjusting brake light switch
Remove brake light switch to adjust.
‒ Pull connector off brake light switch.
‒ Remove brake light switch by turning to left through 45° or 90°(depending on switch).
Installation of former brake light switch version (with corners)
http://img14.imageshack.us/img14/4396/n4603103zt.png
‒ Before installing, pull out plunger completely.
‒ Press down brake pedal as far as possible by hand.
‒ → Guide switch through fitting hole and secure by turning 90°to right.
‒ Release brake pedal.
‒ Fit brake light switch connector.
‒ Check function of brake switch.
After adjusting brake light switch, ensure the brake pedal is in the end stop position (released position).
Installation of new brake light switch version (round)
http://img14.imageshack.us/img14/6497/n0105322ey.png
‒ Before installing, pull out plunger completely.
‒ → Guide switch through fitting hole, press against pedal and secure by turning 45°to right.
‒ Brake pedal must remain at rest position (not depressed).
‒ Fit brake light switch connector.
‒ Check function of brake light.
After adjusting brake light switch, ensure the brake pedal is in the end stop position (released position).
