A number of things could have happened to your drones. You could have turned to sharply and they blew up, they could have landed but on the grass and you just didn't see them, an enemy fighter came in and took them out (which I highly doubt but seen happen before), or like CAP1 said they disapeared into the ground for some odd reason. However, if you landed on concrete, then you can't loose any perks, even if your drones aren't on hard ground. 3 perks is all you earned on that bombing run, you didn't loose any.