If I draw an enemy that is discarded because it cannot spawn, do I draw another encounter?
In the situation where no location in play is a viable place to spawn the monster, then you have lucked out and the card is discarded without drawing a replacement card.

From the rules reference (page 19 under Spawn):


  If an enemy has no legal location to spawn at (for example, if its spawn instruction directs it to a specific location that is not in play, or if no location in play satisfies its “spawn” instruction), it does not spawn, and is discarded instead.


Nowhere in the rules does it state that a replacement card should be drawn for the discarded monster.