Despite your hardware being perfectly calibrated it's quite hard to keep up with the rest, especially if you're not too experienced in formation flying. A good mission leader should always pull his throttle back a little because every slight movement the followers do will slow them down ever so slightly, yet cumulatively. Thus if you're the last in the row you're prone to fall back. It gets worse if all of the missionaries should look for enemy activity below them. The continuous sideways tilting will soon eat your energy with the said result.
So it's not your hardware. Next time it happens ask the leader to slow down about 10% and if he won't, join other people's missions next time. The one flying straight should fly slower than those flying zig-zag behind him.