It sounds like either the 2 residences or 1 of your sewage plants is on a road that doesn't connect to the rest of the city. The services flow along the roadways, so if you just built a little road that isn't connected to the rest of the roads continuously and placed youtr sewage plant there, it won't be able to connect to teh residences.
Easiest way to check is to touch and hold any of your sewage plants. A brown set of lines will appear showing the sewage flows. make sure all of your sewage plants are hooked into those lines, and that the 2 residences are on a road connected directly to where the lines go. If not, you have a missing piece of road that isn't connected to the rest.
Yeah idk if this is a bug or what but in odd spots I am not hooked up to my sewage or water; while the places next to them are. I am currently at level 19 and pretty hooked, but this is making it not fun for me.
No the Sewage plant need not be next to a house... Just make sure the roads are connected, the one to which the sewage plant is placed and the one you have your house....
