Tough to say for sure without knowing what settings you have checked, but here is something to look at in case you have not.
The settings in the screenshot below can be found under the RULES tab in your League Settings page. Circled one will determine how many rounds of available players there will be. This will work with both fictional and historical databases with one exception. If the available pool is set for 5, it will not matter how long your draft is... you will only have 5 rounds of players available.
The exception:
If you are playing historical, but not random debut... you will only have as many available players as debuted that season. This happens often pre 1950 where you may only get 3 to 5 rounds worth of players assuming you are using the default number of teams for that season. If you double the number of teams via expansion, you will only have enough players for half the number of rounds.
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