Pitches are probably the main culprit, and once you get the fit right, try Magic Carpet from Ron C. It is by far the best thing for the thumbhole; better than the foam thum pillow. When you first use one, it can cause a redness of the back of thumb, BUT that is just a breaking in stage. Keep us informed.
dave
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If You Are Not Using a BuzzSaw, All You Get is SAW-dusted!!