Janis,
Based on what you are saying this is what I assume:
For lower volume short oil patterns you are throwing a Storm Natural and you have a Tropical Indigo but it is too much back end for you for short oil. You say that the Natural works for you early on but as the oil dissipates it misses the pocket - presumably hooking too much?
All of that as assumptions you must be a down and in bowler and probably fairly slow speed. You have 3 real options if all of my assumptions are correct.
The first is to learn to move your feet and target into what oil is left so you can keep using your Natural. Any coach you can find should be able to help you with that.
The second choice is to polish your Natural so it gets through dry lanes longer. That may be your cheapest solution and will get you an extra game playing the lanes down and in.
The last choice in my opinion would be to buy a plastic ball or an even weaker urethane ball. The best option for that would be a Polar Ice.
I would suggest you find a coach or even your ball driller to watch how you throw the ball and suggest a solution. Keep in mind the two balls you are carrying now are considered dry lane balls to begin with, so going to anything weaker should be what you do after you try to improve your game first.
