After reading the FAQ and info and testing some stuff, I found it. Your slice has to be in the rectangle in it's entirely (so with big slices, you have to be in the middle of it), and you need a new blue block.
So, if you have an area which is long from east to west, it's more difficult to get, than a north-south based long slice.
The airport is just a bit to wide, but I can slice a bit of at the most eastern point. Although, I have to do it on foot wandering through a forest and swamp.
The part between Zuidbroek and Sappemeer, that's gonna be a difficult one because of the highway, canal and railway, having their crossings at the same point, and complete west/east wide, too wide. And the Zuidlaardermeer (lake), the Paterswoldse Meer (other lake) had bikelanes, roads etc alongside the water, and at some points the form of the lake was such that you had the entire lake in the rectangle. For the Zuidlaardermeer to have that, the only place is in the middle of the lake itself, and then you cannot even see the roads surrounding the lake.
Well, perhaps I should walk around the lake or something and then hire a boat.