Here's a response I got from Yahoo regarding players missing from database:
Thank you for writing to Yahoo! Sports.
We are working to expand the Available Players (in-season) and
pre-ranking (pre-season) lists. There is no set timeframe for the
addition of players to Yahoo! Sports Fantasy Baseball. Players are
added to the game as soon as possible during the season.
All player requests are reviewed and players will be added to the pool as soon as possible after their Major League debut (this requires approval from a third-party, so we are unable to comment on the timetable for addition of any specific players).
Please note that all players added to the game are made available via Waivers.
** You will be able to find the list of players that are added to the
game by clicking on the "Players" link from your league home page.
Then, select the "Status Changes" link to view the latest updates.
Players who show as "Activated" are available as free agents, as they already existed in the available player pool.
Thank you again for contacting Yahoo! Sports.