After take off, make sure of your alt AGL. Your plane must be over 200ft from the ground in order for the drones to start auto formation maneuvering.
You have to remember that even if these are just drones, their flight performance does not exceed that of the real plane. It's not going to make sudden superspeed bursts and catch up with the lead buff. So, after you make sure of your alt, if you see a drone lagging behind chop throttle and go as slow as possible, until the drone forms up on you.
In some cases, the drone not only lags behind, but is way below your plane. This is usually caused by irregular/unstable take off process where one of the drones 'snags' a hilltop or some other piece of ground when it is trying to take off. In this case, slow down as much as possible, and lower your altitude to the level of the drone lagging behind, so the drone does not have to fly uphill to try and catch up with your lead buff.
Now, if you can't do that because there's a hill or a mountain in your way, then it means you chose a wrong place to take off from. Get up from a base more fit for a bomber to take off.