Why don't some players appear in the player pool when a positional max is set to zero?
League Managers have the ability to set a maximum on the number of players eligible at a given position which a team can own at any one time. This is intended to prevent hoarding of positional players, especially in deeper leagues. Setting a max of zero (0) on a position implies that teams can't own any player eligible for that position and, effectively, prevents those players from being drafted or added via waivers or free agency. This is a common source of confusion. If you are in a league, for example, where DH-eligible players do not appear in the player pool, even though the same players may be eligible at other positions, it is almost certainly the result of your league having a positional max of 0 on DH. The number of starters and the positional max for each are highlighted on the Rosters tab of the League Settings page.