To add to what Bob and 'Bones have said, it can be any number of things.  However, it typically doesn't come down to just one thing being different, but a combination of things: Speed, release, extreme line change, and so on.  Most bowlers can change any one thing and still be in a relative "comfort zone", but when 2 or 3 or more things need changing all at once, the "A game" goes away pretty quickly.

For me personally, changes in speed and changes to extreme lines do not detract greatly from my game.  Yes, there is a speed and line I'd like to use all the time, but I don't lose much in performance outside of them.  When you add a third element of change, things start to get tougher pretty quick.

--------------------
Kill the back row (or maybe this should read "make your spares, dummy")

Me:

A game: playing outside throwing harder with speed and roll (Parker Bohn style sort of)

B game: right off of the corner when they are dry with more speed and less hand (Walter Ray style again sort of)

C game: (yes I go that far sometimes): Inside many revs as I can get with speed, more axis rotation and not as far out (Jason Couch only much slower and not as many revs)

D game: drink beer and throw at the head pin.

E game: drink more and bowl right handed

F-Z game: do shots and give up.

Laner
--------------------
Hell hath no fury like a woman scorn for Sega....

to me there is no a and b game in bowling it is just changing to the conditions. just cause you don't crank doesn't mean if the condidtions call for it you cant swing the ball a bit. Also you can have a cranker straighten it out and shoot down and in. to succeed now a days you need to be good at adjusting. This is only my opinion.
--------------------
Oil is served Best with fingers!
Why does the 8 Pin laugh at me!

Sheppy

Interesting sometimes to stop and think what is your A or B. Mine has actually changed over the past couple years, but thanks to a different center I am subbing or bowling my old A is rearing its head again.

When I was at my peak my A was playing around the 5 board, maybe as deep as 8 swinging to the edge. Over the years and with change in equipment and conditions that A moved deeper and deeper. Now that I am playing both the gutter and as deep as 4th arrow or more I can see a difference in several aspects of my game. When faced with the gutter shot or close to it, I am more in line with the lane (obvious to play the angle), but also get much more roll on the ball using different hand positions. I also have a much more solid push away and longer, smoother armswing. Ball speed tends to be higher, but that depends on the volume of oil on that night. I have even been able to move up the boards with success on occasion, something I never could do before.

Move inside and my mechanics also change. I will tend to have a waggle(similar to Chris Barnes) to open the sholder. The wrist is typically in a cupped position and the push away more of a downward motion. Armswing is a little more muscled with ball speed reduced by the shorter armswing. Recently the deeper angle ends up being my A game only because of drier conditions, especially second set of travel league. When they get really dry the speed becomes accelerated,but more from muscle than changes. Had the choice the old favorite of playing somewhere around 6,7,8 and swinging out to the 2 board would be where I would go. Something I might not have said just a few months ago. I has felt good to roll the ball instead of throwing it for a change.