I had that problem a long time ago (wrapping the ball behind my back). I was (and still) am, an old time stroker--- minimal forward and side tilts, square shoulders, no crossover step, pushaway directly in line with shoulder, posted release. In my case,
the cause was due to my rotating my bowling hand (right hand) counterclockwise on the backswing. This shifted the ball's weight slightly inward, causing my swing to
"tug" behind my head. Fortunately, on the downswing I would rotate my hand counterclockwise again, but this was causing way too much movement in the swing.
Stabilizing my hand from stance to just before release, eventually corrected the problem.
I agree with the suggestions from the various posters. The thing to remember is that
there can be many different reasons why your armswing is not straight. How you set up in your stance is critical, as JustRico pointed out. There is a lot of truth in the statement that, "if you do not start right, it's hard to finish right". The direction of
your steps, as well as the position of your bowling shoulder from start to finish play
key roles in determining the path of your armswing.
Basically, I found that your ball alignment in the stance, and in your pushaway should follow in the same direction as your key (pushaway) step. If you deploy a crossover step, then your pushaway should follow in the direction of that step. If you
do not cross over as I don't, then the ball should be aligned and pushed in the direction of your bowling shoulder. Classic strokers such as John Handegard and Mike Durbin are good examples of the latter. Once your body is in motion, it is important that your bowling shoulder remains "silent" --that is, there should be no excessive
movement of the bowling shoulder . Pre-setting the shoulder in your stance and maintaining it throughout the approach will help your to maintain a more consistent
swing plane shot after shot.
Good luck!