Steve...That is a great exercise (Joe has lots of great stuff to learn)! You can do the same thing with a couple of OB's. Jerry Briesath shows it on his new dvd set.
1. You may have a perfectly straight stroke now but grip issues that cause erratic hits just slightly enough to thwart you, or you might have a decelerating cue stick or aim is at issue (you may need eyeglasses) - or about ten other things.
2. Your stance may be at issue. If your cue tip is further from the cue ball at address than the width of one chalk cube, you're too far from the ball. On the other hand, it need not be right atop the cue ball--many players use a little gap to get their stroke right before impact. But adjusting the distance with your shooting hand is a no-no, I'd need to adjust your feet.
3. Etc., etc.
Send me a video and if you can't, photos. I'll give you some advice for free while you're saving pennies to visit with me or Mr. Lee.