Make sure that your spawning areas are completely dark and that your canals are wide enough to allow spiders through so they don't clog it up. Also make sure to light up all areas around the grinder so that mobs do not spawn there first. Keep in mind that you need to be at least 28 blocks away (I believe) for the mobs to spawn, but no more than 150 away so they don't despawn.
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Yes hostile mobs spawn up to 128 blocks away from you, so they could be spawning in caves underneath you rather than in your grinder. They have to be atleast 24 blocks away to spawn and between that and 32 is the sweet spot where mobs dont despawn