EA placed a CAP (Create A Player) restriction this year in M17. After EA's last roster update, I had a total of 36 create a player slots available to use. I used one of them to re-create Marshawn Lynch. I used the remaining 35 on Round 1 rookies and 3 rookies from Round 2. After the last CAP player is used, anything you create after that disappears from the roster and does not save.
In order to continue to add rookies to the roster, you have to overwrite and utilize rookies from last season that are unsigned in free agency. 95% of the time they're faceless which helps. You have to go through all of them and match up height, weight and appearance to the best of your abilities in order to match them to a drafted player from this year. I then go in and re-edit their name, hometown, college and equipment. Follow it up with edited ratings and player traits. It's a drawn out process but that's why it's fairly impossible to do all the rookies this year.
Hope this helps?