If you don't spend time teaching box out technique and ball pursuit you can't expect it to appear by magic during a game. Many coaches teach technique but too often fail to give their teams practice opportunities in reading where a ball comes off the rim. It amazes me how many young players cannot read where a ball is coming off the rim. As a very young player this is one of the things I was taught and to this day have contributed to my ability to read a ball even with limited natural athletic ability. Finally pursuit is critical to rebounding.
Many players play flat-footed a great percentage of time on the court.
This definitely affects a players preparedness. Teach that skill by observing players who fail to stay on their toes during drills and in the game. With that said, here are a few additional ideas that might assist you in getting your team to rebound better and play more aggressively. This sounds pretty stupid, but I worked with a coach who put white tape on the forehead of each player and told them it was their rebound radar. It didn't make them jump better but it did bring constant attention to an aspect of the game to which they were oblivious.