bayboy

You are probably trying to dock with the docking axis along the prograde/retrograde vector, or the radial vector. As the two vessels orbit, their relative position to each other changes. E.g. if one is "on top" of the other, after 90 degrees of orbital rotation, they will be "side by side", when viewed from above. But if you align them "north-south" before docking, they stay in that position throughout orbit. I think.