It's pretty simple, when a ball strikes the pins you deal with 2 main areas of deflection (ball and pin deflection)
Ball Deflection, the ball strikes the pins and is deflected off it's roll path by the impact. This should occur when driving through the 1-3 pins which results in the ball physically contacting the 1-3-5-9 for right handers or the 1-2-5-8 for left handers.
Leaving a 9 pin is commonly caused by a ball which is still driving hard into the pocket, it doesnt get the deflection off the headpin like it should do and it continues on its direction across the deck. What you end up with is a ball that hits the 5 pin solid and continues driving through, it goes right past the 9 pin and leaves it cold. Same thing for the left handers but in reverse, it drives through the 5 and leaves the 8 cold.
The total reverse can happen though, leaving a cold 8 pin is commonly a sign of a ball which is weak on it's entry (loss of energy) and has been deflected too far when it hits the pocket. The ball doesnt drive though the headpin and takes the 5 pin lightly then follows through to hit the 9 pin solid. You need the deflection on the 5 pin to carry the 8 pin (reversed for left handers), a ball that is lacking in energy usually produces this result.
This is where angle of entry plays a part, it isnt however the be all and end all, low ball speeds and low ball weights can have the same effect. A good strike is when a ball hits the pocket, retaining enough energy to drive through the pins, deflect the 1, 3 and 5 pins, then having the ball deflect enough to continue through into the back row and take out the 9. It doesnt really matter how you do it, you can throw it dead straight, but you need a combination of good pin deflection and good ball deflection to carry a strike.
Too little ball and it rolls dead and leaves you a stone cold 8, too much and you can drive past the pins and leave a cold 9 or send the pins flying (you want to try and keep the lower on the deck to maximise carry) but leave the corner pins.
As for the other question, just because a ball is covering boards it doesnt mean it is retaining its energy. If you think you are nailing the pocket and leaving a 7-10 (aside from any terrible rack) you have to examine whether your ball is rolling out and it is hitting like a wet sponge. You can get someone to watch your shot and watch the reaction of your ball mid lane and then when it flips to see what it is doing, you can generally see what happens when your ball hits the pins which will tell you how well it is driving through when you reach the pocket.
Of course, you can be really unlucky with all of it and catch a bad break, there is a science to our game but we still need a little luck thrown in for good measure
