What was the lighting condition? And what were the ISO and shutter speed and apertures? Was it the 1/100 second f/5 case? Was the ISO too high trying to deal with the maximum aperture of the zoom? If the light was less than bright sun, that would be my bet.
Casual snapshots might be one thing, but one very main rule for "portraits" is to
always stand back at least six feet, better eight feet, and I like ten feet. Use the lens focal length that allows that. Where the camera stands determines perspective, which for portraits, means how large does standing too close make the enlarged nose look?". Subjects tend not to like that look.
For head and shoulders shots, 105mm lenses were considered portrait lenses for 35mm film (and FX), simply because it forced us to stand back that far. This equivalent would be a 70mm lens for DX. A wider view than head and shoulders needs a shorter lens, but not 35mm - which would be what? 3 or 4 feet? Too close. I always thought 50 or maybe 60mm was too short for waist up DX portraits, I had to stand too close. Unless you're doing like full length portraits, 35mm lens would be considered too short for DX portraits. That's what the 85mm f/1.8 is for.
So the zoom wins that advantage.
But for a f/5 lens, maybe the issue is the need to provide more light for the portrait, so it can stop down a couple of stops, to get it up to f/8 or f/11, at a decently low ISO. If an issue, this could use flash, or a reflector, or some different situation, less deep shade. For example, if under a tree due to the bright sun, that's good, but reposition subject more out at the edge of the shade, not back in the deepest shade.