Only Half of Function keys wont work

I have a mac mini with apple wireless keyboard. I noticed that the F1-F6 of my keyborad doesn't seem to response.

I unchecked the "Use F1.. as standard function keys" in Sys Preference.

The F11, F12 works normally as volume control.

I dont know if this is mechanical or software issue. I can't quickly call mission control with f3 on my desktop. I used to use a trackpad (on which I used the gesture to call mission control) but now I switched to a normal mouse. I need to use mission control sometimes.

Any ideas? Much appreciated!

Thanks!