A face in Sketchup has an orientation. Normal to the face may be oriented towards the outside (towards the sun in our case) or towards the inside (of the building in particular).
During SunExposure calculation, if the normal is oriented towards the interior, the calculated face is inside the building and never “sees” the sun. The face visible from the outside then appears not calculated.
To calculated that face correctly, you need to “invert the face” so that it “sees” the sun and relaunch the calculation for that face.