If you are setting up Keepers, and cannot find certain players, 99% percent of the time it is because you began a practice draft and selected these players.

If this is the case, make sure to RESET DRAFT, and then those players will return to the player pool.

If this doesn't resolve the issue, the other issue is you may not have the correct positions setup for use, via the SETTINGS area. Make sure to review which positions you have designated to use, by clicking on the USE POSITION toggle.