Well I tried few more times with all the things checked (NOZ 0/GEAR UP/TONE not selected etc.) still can't fire the missile.
Only weird thing is that sometimes(not always, about 3 of 10) although the crosshair shows the missile locked but the crosshair is offset from the target a few distance away.
So the problem is with my client then. I guess, maybe in my client, the seeker is actually locked somewhere on the ground instead of target's heat source? I was told by the system that missile is locked and can be fired but actually it's not locked yet. Like71st_Mastiff said:
But this is strange because I can't fire missile on moving targets either although I'm sure that seeker is tracking correctly.