I've mentioned this before. The misses occur only on the mobs that spawn anywhere around the bottom half of the map. Enemies that spawn on the top half, hits always land. Once they go below the flame, the initial hits just don't land.
Its why I made a kinda band-aid suggestion of putting the flame in the bottom center of the screen and to not allow mobs to spawn anywhere except from the top half of the screen to avoid this bug. There is just some issue with the way the enemies spawn and path from the bottom that makes it so they avoid all attacks at a certain point. Usually most noticeable with the red worms from 40-60 but can happen at any time.
Edit: Just saying its not a build thing. Its just a pathing/speed issue with mobs when they spawn from the bottom half of the map.