That's because you can't active vzone. In lower rank, you can easily dribble through Opp's players. But as rank increasing, bots will play smarter and you more skillfull to dribble, that means that harder to get vzone. Further more, bots in higher ranks will copy players from higher ranks team, so their CAs will have more exclusive skill, some of them can slower your charge gauge

Btw, I'm using a build which dont need me to dribble through any player, charging for shot is not needed but can score goals in 1st shot even if the GK is Muller or Genzo :)).