This is probably happening because when you try to make "a really good shot" you are grabbing at the ball alittle in your follow through. If the shape of your thumb is oval, you are far better off with oval instead of round. If it is the natural shape of your thumb, then it should be very comfortable.

Many of us, when we try "really hard to make a good shot" we tense up a bit, and that creates the tendancy to grab in the follow through. Bowling is all about relaxing, and a muscle free swing----or atleast as muscle free as possible.

I know exactly where you are coming from!

Is it a premade oval?

For me those are just too sharp of an oval!

I don't like the sharp corner.

Instead I had an IBPSIA member drill one using an ovalmatic jig.
I then had copies made.  Taped it two pieces short and made copies of that!

Now I'm happy!

It could also in your case be too sharp of a side bevel.
Much pulling of a ball is caused by not enough side bevel!

REgards,

Luckylefty
PS another way to do what I did is get a round insert wide enough for your oval thumb.  Tape very precisely the front and back leaving space for a least one piece of tape in the morning(when most peoples thumbs are biggest).

Then go have a copy or an exactacation made after testing the perfect fit by throwing!  
PPS by making perfect taping I put a 1/2 or 3/4 inch piece in center of thumb front.  Then two 3/4 meeting at center of thumb. Then a 1 inch piece of tape.
This gives me a somewhat flat surface in front blending in to the nice roundness I like at the corner of my thumb from the original thumbhole insert.
Hope these ideas help.  Above taping procedure depends on width of flat surface of thumb.  If you only use a 3/4 inch piece of tape then use 1/2 inchers under the 3/4.