Adjacent spots 'share' spawn points, so if people in nearby areas are also killing mobs, they will respawn in the surrounding area. If you are the only one killing in the area, they may have just leaked over to the areas to the side and you need to go kill it yourself or park yourself in an area next to someone else.
This is an example from b1 - the black line shares spawn points, and the blue line also shares spawn points. The only reason why I have a speculation of this is because there was a night where the channel was completely dead and I the spot you see me at in the screenshot was completely empty of mobs, but the lower area had 3x the normal density. As soon as I cleared that area out, the spot I was originally in almost instantly filled back up.
To further add to this, as the area in black is where I am currently farming and all 3 spots are empty at this moment (someone is clearing the blue line's corner area):
These two are screenshots of when the area that I farm at usually (the 'corner' of the black line) started to lose density - when looking at the bottom part of this 'black line', you can see that the mob density looks abnormally high. However, once we clear that bottom area, we get this:
The spawns in the first area get almost immediately repopulated as soon as cleared out the bottom area.
This one is another location in b1 that the 4 'circles' have shared spawn points. There are definitely other locations as well and it is safe to assume the first floor acts in a similar way.