Flash problem

Hello again Oracle(s),

I have a K20D, on a recent outing to a dimly lit cave I could not get my inbuilt flash to fire, I tried all settings; manual,auto,SV,TV,AV. I also checked to make sure that flash mode was on in 'Fn'
However, I did get a few halfway reasonable shots by upping the ISO and slowing the shutter, albeit rather shaky.
Today I tried at home pointing the camera into a dark corner and the flash fired in all settings with flash mode on.
Am I missing something?

Did you wear a hat in the cave? If so, if the flash touches the cap and is pushed slightly down, it won't fire.

Other than that, I can't figure anything else.

I was thinking if the AF didn't get a lock then the shutter won't fire and no flash. Switch to MF?

Thanks chaps, in fact, thinking back, both of these answers are actually relevant.
Makes sense, much obliged.