Advice

I work with a team of 12 and 13 year old girls who at tryouts looked amazing as individuals and as a team on paper looked great. But now on the court they are struggling! They seem to lack confidence in themselves and just aren't pushing the way we know they can...the way we have seen them push when they have played on other teams. They are all mostly quiet girls but very competent ball players...just at a loss as to how to get them all out of their shells and pushing more. Like I said, they should be dominating the teams we are playing but they just aren't! At a loss and would appreciate any insight you may have.

Is it an offense or defense issue? Turnovers? Rebounding? Effort?

Rebounding and turnovers mostly

Sometimes the answer is so obvious you don't see it. I do this all the time. If rebounding is an issue, emphasize it constantly. Sounds overly simple but that's all you can do...

Reward the effort. Emphasize rebounding in almost every drill, track stats, compliment and reward the rebounding effort, and make it a prioritize.

Turnovers... where are most of them coming from? Dribbling into traffic? Bad passes? Maybe they just don't understand the offense well enough yet?

Mostly bad passes.

Is it an accuracy issue, skill issue, not familiar with offense, mostly under pressure, mostly during press break, not meeting passes? Find some drills that address the particular passing issue(s).

You can find some good passing drills here:
https://www.breakthroughbasketball.com/drills/passing-drills.html

It might be a matter of spending more time on the offense and adding constraints to your scrimmages during practice.

I'm a big advocate of making practice harder than the game. That means, allow more unfair pressure in certain drills and scrimmages. Think 3 vs 5 or even 6 in some cases. No dribbling.

Play "5 good passes". Teams of 3 if you can do it. No dribbling allowed. Half court. Team 1 plays against Team 2, Team 3 is on the sideline ready to come in. Team 1 has the ball and needs to make 5 good passes to win. Team 2 is very aggressive on defense and is trying to get a steal or knock the ball away. If that happens. Team 1 is off the court, Team 2 is passing and Team 3 comes in from the sidelines. This moves fast. The first team to complete 5 passes wins, losing teams have consequences.

Rebounding. I'd emphasize blocking out in a big way. Find drills on that, make it a big deal.