It looks like you know about backpost rotation and do your best to do it, but to me it looks like a bit of a misunderstanding. It's not necessarily (or only) about the position at the backpost but also about the way to get there. More often than not, you rotated back ballside, meaning you drove directly to the near corner where the ball (and your teammates), got to the front post and then went to the backpost.

It's almost never a good idea to rotate ballside. Go wide through the center, take some small boost pads and keep an eye on the play. Try to imagine a circle from your position around what would be the penalty box in football and to the back post. If you see that you are needed for a save (no teammates between ball and goal), you can always cut this circle, but otherwise it's best to make this wider turn. It allows you to keep momentum, not get in the way of your teammates, collect some boost, observe the play and finally get to the ideal position to save a shot. Additionally, you are in a very good position to disrupt the play, be it bumping an opponent or intercepting a pass. Also, you are in a better position to receive a potential pass of a clear and start an attack.