First question is what do you do, if anything, to clean the ball between sessions. If you don't clean it at all then you are probably running into oil saturation and that is why the ball will not react the same week to week.
Next question is what do you use to clean the ball after each shot? Does it truly clean off the excess oil or does it just "push it around".
Example: In all the balls that I use during the season, and I bowl almost exclusively on Sport Patterns, I clean each ball with a green scotch pad and 70% rubbing alcohol. note: you would be truly amazed by the amount of dirt and oil that comes off each ball during my cleaning sessions. For my waxed balls I use a clean cloth and the rubbing alcohol to clean up the rubber marks from the ball returns and the track marks on the ball, but I also clean them with the green scotch pad and alcohol about once every three weeks and then put them into a ball waxer for a couple of minutes to restore the shine to them.
Doing the above on a regular basis has been very successfull for me in extending the life of the ball to the point that in the last 10 years I have NEVER had to get rid of a ball due to lack of production.
Just a thought... Hope this helps you out...
Gallandro