Hi there Ihope my experience help you...
here goes...it happens to me too so after some time of that i´ve started to fly well pass the place where there sopposed to be (the target) and it magically appeared, but in most cases this is caused by the "front haze" before you choose a mission place your mouse coursor and read what reconoisance level the sector has.(none, medium, high) In some cases for example an invasion starts if you choose a case very close or itself to the invasion one, or far away, perhaps you should do a recon flight before start or perform any mission.
if you do that you`ll notice that the missions aviable are not the same...so, there`s your answer.
Also if you see that the status says "none" try a front-line case near the one you choose.