If your players are being brushed off the ball you need to play games like 2v2 or games with overloads so they have more of the ball and encourage them to win it back if the lose it. I wouldn't say they need to be more aggressive just less accepting when they have the ball or when they lose it – guard it and win it back in 6 seconds. Games like 2v2 or 3v2 can in themselves mean the girls will not want to let their team mates down in such small sided games.  Try playing these games and you will see that on the pitch they are less likely to lose the ball and more likely to be keen to win it back!