I think these are the 3 reasons why someone may struggle to throw straight at 10 pin:
1) They don't have a straight armswing
2) They don't have good feel for the ball. So even if arm swing it dead straight at 10 pin, they jerk the ball left or right at release, because of their fit.
3) Over time, minor changes to drift and arm swing, cause a need for realignment. About a year ago I was missing left. Used to stand on board 31, had to adjust and stand at board 31 1/2 since for some reason.
The amount of hook you'll get from a spare ball on anything except the shortest sport patters is so little, that should not be the cause of misses.
Just practicing shooting the 10 pin is a waste of time, if not correcting the problem. What has helped me if taking a game or two to just practice spares, focusing on releasing ball off my hand the exact same way each shot. As the only reason I ever miss is my #2 listed above.