Spotting question

[FLG]

In one of my games i have airpower available but when i try to use it on any enemy unit i am told that the target is unspotted!

How can this be when the enemy unit is adjacent to my units and shooting three shades of brown stuff out of me?

It is probably unspotted by a unit of the same formation. It sounds like the air unit is attached to a specific army and only a unit of that army can do the spotting. Check your OOB.