PROBLEM
Railing won't host.
CAUSE
Railings will only host to Stairs, Ramps and Floors.
PROBLEM
Railing won't host to a floor.
CAUSE
Railings won't host to:
Floors with modified points or split lines
In-place families (even if Floor category)
SOLUTION
Use a Ramp or Floor with a Slope Arrow.
PROBLEM
Railing won't host to a stair.
CAUSE
If the railing will extend beyond the view's Top View Range Revit creates a flat railing.
SOLUTION
Make sure the Top view range is above the top of the stair the railing is hosting to.
Note that if the stair is multi-storey the top view range may be quite high.