Why doesn't my screen lock work (HTC Wildfire)?

I have a HTC Wildfire with android 2.2 (I can't remember if this problem started right before or right after upgrading to 2.2, but the upgrade was done recently and so was the first time I got this problem). Sometimes (and it seems completely random) the screen lock function never works. When pressing the power button and pulling down the clock it tells me the screen is locked, but there is no lock there! When I pull it down I get to my home screen instead of the screen where I need to enter my lock pattern.