Thanks for the pictures. Now I see what's wrong.
Your problem is the flushing system that has water flowing OVER the drop hole. Mobs swim up in water, so with water over the drop they won't fall through. You need to either adjust the size of the room so that the water stops at the edge of the drop hole, or use signs to block the water from covering the hole (assuming signs do actually block water in XBox/Bedrock edition).