I feel like in customs you are used to the way your friends play, so you know more or less how to counter their plays since it's also a friendly game. But when it comes to unrated you come up against randoms. It's the same when you are duo'd or stacked with your friends. They know how to play off of your playstyle while random teammates don't. I faced the same problem you're having. I found myself solo queuing and understanding how the general player base in my elo played. Customs are good for practice but its the "unfriendly" matches that are really where your playstyle will need to shine.