When I first heard that there are people traveling backwards with the kick, I tried the following:
Superman Glide and then flutter kicking. Only small movements in hips and knees. My ankle flexibility is not the best, but I can keep the feet in an almost straight line with the leg. I move forward, slowly, but steadily. Then I start to flex the foot, meaning keeping it straight but slowly decreasing the angle between front of foot and shin from 180 degrees down to 90 degrees, ankle movement, basically. My forward movement gets slower and slower and when the foot forms a 90 degree angle to the leg I start to travel backwards! Voilą, there we go. Quite funny feeling. Slowly straightening the foot again and I start moving forward again.
So, one option, what are your feet doing?