Usually the reason is the mob you can’t find have walked off to the platforms above the water you do not clear, and are not moving. This gives the appearance that they are normal spawns on one of those 2 platforms people usually bypass. If you cycle through those spawns you will usually find the mob you are looking for.
Another spot runners path to is left and around the corner of the narrow overhang facing on the outside of the building just before you leave the water area.