Hi Phililp, the sides you have there did not end up being planar, if you switch to the Right side view you can see:



Notice how the profile is kind of curved and shrinks inwards near the bottom?

It looks like your middle profile curve is not quite as wide as the others, so it does not end up tracing out a planar shape along the sides.


So instead of Construct > Planar for those particular areas you will need to use some other technique, like for example select these 2 edges and then run Construct > Loft :



Or you could also use sweep with those 2 edges as the rails and the little line as the profile curve, or you can use the other technique I showed above where you would cut in those side pieces with booleans instead.

- Michael