Honestly, you have to change the angle of entry into the pocket. You could move up a little bit on the approach or move back a little.(maybe only half an inch) You could also try moving a little left or a little right of where you were standing. Again, this should not be a big adjustment. These little tricks might be just enough to get that 7 pin out. Good luck!
Ah yes. To be honest there really isn't too much you can do. Being a right-handed bowler, I kow how you feel (me leaving the 10-pin). All I can say is that you can move forward or backwards about half an inch. The only other option is to change balls and change lines, but thats for desperation when no other option works.
I just came back from bowling and I kept leaving 10 pins (I'm right handed). I moved back 2 inches. I'm telling you it helps. I gives the ball more time to enter which my reflect on the 8 pin more which will help you to knock that stupid 7 pin out. Hope it helps!
Your problem with leaving the 7 pins as a lefty would not be the ball. The problem would be that you are hitting either light or high (in the pocket). Try to make a move about 2-3 boards right but keep your same target. Also you may try moving a bunch of boards and then if that does not work move your target in 1 board at a time. Try this and let us know how it did for you.
Moving lft,rt,up,back works great on wood lanes.
On Synthetics however i would suggest a different approach.
I suggest altering the speed slightly by holding the ball higher or a bit lower. Also changing hand positions on the ball slightly will often either flatten out the angle a bit or create a little more angle depending of course on what type of corner pin leave you are getting (swishing, flat, high, etc) spread your pinky out more or bring it in tight against your ring finger.
Sometimes a slight loft will acomplish this as well.
Again, you can try altering the surface of your ball slightly.
hope this gives you some ideas to help your game.
remember though if you plan on altering the surface of your bowling ball, you can only do that during your practice. So before you bowl and plan on making surface changes, do it before or during practice, but you can not do during actual league play or tournament play.
April makes very valid points. I have not experienced a difference between wood and synthetic lanes as Mr. Harris has noted, though he offers some good pointers as well. I'm a big proponent of small movements on the approach: left , right, forward, and back. I'm also a big supporter of making slight adjustments with hand position.
Personally, you need to be sure you're being honest with yourself as to if you make a good shot in the first place. I have many shots that people believe are perfect and end up leaving the 7 pin (I'm left handed). However, I know on these shots I've typically rotated my hand early which changes the angle of axis rotation slightly. I know it when it leaves my hand and can typically watch the 4 pin shoot right around the 7 pin. Just some food for thought...
Sorry if i gave the impression that you should change the surface of your ball during sanctioned play, this is against USBC rules. I assumed (my bad) you would know that this was something to try during your normal practice sessions.
My experience with making slight adjustments on wood vs synthetics may have more to do with bowling here in the Midwest. From what I've heard and experienced over the years, the conditions tend to be dryer here than on the coasts.
I've not bowled in the east, but in Ca. this seemed to be the case. Don't get me wrong, i agree with Coach Frye completely about making small adjustments, and being sure you are making good shots. The point Coach Frye makes about hand position is more to the point i was trying to make. Anything that slightly alters the angle of axis rotation,speed, or break point, can make shots that look great start to lose carry.