Let your girls know that pitching is not a position everyone will play. I like to be fair to my players. I also like to be fair to the team as a whole. A girl who struggles in the circle is not helping the team. As you know, pitching is not easy. It takes more work away from your standard practice times to get better.
The best way to make sure they are working on their own is to have a pitching contest. You can hold this quickly during the practice before your next game or even on gameday if you have time.
After each is warmed up, they get to throw you 20 pitches (you can pick any number you like). Most strikes pitches the game. Second place would be the first reliever if needed and so on.
This will make them want to improve on their own so they do better next time. If they work for it, they will get it.
We made this game up a few years ago. We like to play it at the end of practice if we have time. The kids really enjoy it.
You line up your players in an arc stretching from thrid base to first. This is the fence. You have one girl batting and one girl on deck (girl on deck should have her helmet on and bat ready to keep things moving). A coach will soft-toss about 10 to 15 balls to the girl at the plate. (soft-toss is an underhand toss from close range, just off to the side of the batter).
Her goal is to hit the ball as hard as she can while still using sound fundamentals. The kids keep track of the number of balls that get past the fence. These could be flyballs to the outfield or hard hit grounders that cannot be fielded.
After the turn is over, the fence players will collect the balls, the batter will gather her glove and fielding mask then move to the position closes to third base. Everyone else who was on the fence, moves over a position towards firs base. The girl who had been closest to third base is now on deck.
Split the team in half. Line up one half at second base and the other half at home. If you have an odd number of players, one girl will have to run twice (first & last for her team).
When the coach yells "GO" the race starts. One girl from each team must run around the bases back to where she started from. Once she gets back to that base, she tags the hand of the next girl in line on her team.
The goals for this are obviously increasing speed but also to enforce good base running. Coaches can watch each bag to make sure the kids are tagging it on the front corner and making good turns.