@_awkwardlezbean
Ok, on the lots that aren't completely flat, when you try flattening them, usually what happens is that the lot will flatten except for one tile along the perimeter on one or two sides of the lot. The rest usually does go flat. I'm not sure what is going on for you. Is it just that one place on the lot where it isn't flat? Can you place the garbage can anywhere else?
One thing you can try is to save the lot to your library. Bulldoze the lot. Check to see if it is flat. If not, try flattening it again. Then place the lot again from your library. That might work.
Sometimes, the house or other objects on the lot will cause a lot to not flatten correctly. Flattening the empty lot gets around that usually.
If that doesn't work. If you are willing to upload the lot to the Gallery and let us know what the lot name is, I or another helper will download it and take a look to see if we can figure out what is going on and how to fix it.
Hope this helps.