Since no one else has mentioned it, here's another possible explanation(/solution):-
The wall is there. However it's hidden by a different element or object in a hidden layer that has higher priority or same layer number, like a massing slab or a Boolean (SEO) object.
If the wall was there and hidden by the floor slab fill, the cursor would still detect its edges when you hover over where it's supposed to be (by turning into the 'edge' Mercedes cursor from the magnet cursor symbol for surfaces and fills). This is the quickest way to tell if that's that's the case.
Alternatively you can check if the wall is there but that section of it is eliminated (by layer order number), by seeing you can select the nodes where it connects with the other two walls. It will show a wall selected but missing that middle section (as opposed to the previous case where it highlights the entire wall with the hidden section as slightly faded out).
The easiest or quickest way to find out if this is the case is to isolate that area in 3D with a marquee tool, turn on all layers, and see if another object shows up where you expect to see the wall. And if that's the case and it's something you don't want to delete, then change its layer number to something different from the one that the layer containing your walls is using and that should clear up the problem.
This happens to me all the time when I forget that in an earlier version of a file I had conceptual massing slabs that I forgot to delete or have an appropriate layer number for and then when I start building walls in place, they disappear in certain views where the layer numbers are all coincident.