I've been coaching 10-14 year-old girls for 15 years. Here's my opinion. The biggest problems comes from social conditioning when it concerns being aggressive. Aggressive females are not looked upon favorably in our society. Especially at a young age.
Some girls have the same natural aggression that boys do and have no problem letting it show. Others have it but are afraid to let it show because they've always been directly or indirectly told that "girls" don't do this or that, but it's OK for you boys to. Most of them have been told that from the day they were born.
So how do you fight years of passive conditioning? Simply telling them it's okay to be aggressive will not do it. They don't know how. What you have to do is condition their thought process in small steps. The place to do it is at practice. An example is ground ball drills. Start by rolling them easy ones. Then start rolling them out to the side further and further, in small increments. Always keep it positive with comments like "Show me you can get there." "I know you can get to that ball, now show me." Always praise the effort, not the result. If you see a girl being lazy there is a way to tell them without calling them lazy. I simply tell or ask if that was their "Best effort?" If they answer "No" then tell them you need to see their best effort all the time. If they say "Yes" and you know it wasn't then tell them the truth. You didn't think it was and you know for sure they can do better than what they showed.
Again, this is a marathon. It doesn't happen in one practice. It is the process of conditioning them into putting forth their best effort and being aggressive is OK. It's acceptable. It's expected. Once you get the whole team with the same thought process it becomes easier for the more timid girls to feel it's okay to expand their comfort zone.
I wish you had posted this 8 yrs ago!!!! I have saved this and if I get back into coaching after DD finishs I will try it.