Well. May be my descriptions not clear.
1) Rules says:
Line of fire: the observer can always fire at all the planes without checking for planes hiding other planes
Problem 1) My observer (not jammed, nor killed) not fired in such position: My 2-seater plane; Behind and left to me - friendly plane; Next in this line - enemy plane.
Explanation 1) Somehow friendly plane blocked LOF - it's against rules.
2) Rules says:
The only exception is when there is no deflection between to two planes and another planes stays exactly in the middle. This can only happens when the enemy plane is exactly behind to plane with the observer and there is no deflection
Problem 2) My plane was shot at in this position: Enemy 2-seater; Exactly behind it tail - friendly to me plane; Next in this line - my plane.
Explanation 2) My plane wasn't in deflection, so enemy's observer can shoot at me. But actually he must be blocked by friendly plane and can't shoot at noone (at me, because blocked; at friend, because in blind spot)
You have a good point, although my origional rulebook does not have that. It only states that you cant fire with another plane inbetween.