It's got nothing to do with your tools, but all with your technique.
If I understand you correctly :
Hold the squeegee at a 45 degree angle, move from top left to top right and when you come to the top corner, don't hesitate in that corner, go straight down with your hand, whilst keeping the 45degree angle. And immediately when you've cleared the top corner, start rotating your wrist clockwise to get a reverse 45 degree angle to make the pass to the left.
This all has to be done smoothly, don't stop in the corner (no hesitating). Do not bump the blade against the rubber in the corner, that might skip the rubber, leaving that diagonal line. Think before you move, then move. Do it slow, speed will come.
Also what is very important is the base distance when your making turns like that, the distance between the end of the squeegee handle to the glass should be round about two fingers. If you have a greater angle, then the rubber might flop when turning the blade. It's difficult to keep that base distance throughout the pane, but it essential for a good finish.
Oh yea, and always cut the edge first, meaning use the tip of your blade to create a horizontal gap at the top edge, bout half an inch wide or so.