Your ball placement will vary in width by roughly your height. Too often, when we try to hit a certain spot, we shorten our swing and don’t let the racket and body follow through correctly.
Hitting down the line or into the alley isn’t a shortened swing. Hitting cross-court isn’t swinging harder or faster.
Give yourself a wider margin, understanding that your natural stroke will move the ball side to side by about the same width as you are tall.
To practice, work on your Alley Rally, pulling the ball back into the Alley with each stroke.