Weight gain in broilers is a factor of feed ,enviriment and genetics interaction. It therefore means you must have all working well to get ideal weight gain.i.e the birds must be real broilers, the feed must be of best quality(energy levels), and the production environment must allow for the gene to express itself( proper flock density, lack of stress like diseases etc). You can start by checking on the above assuming other factors remain constant, check on your feed quality and levels.