Might it be that your right elbow is "uncocked" and locked at address?
If so:
Once you learn to apply Extensor action att address (with a dead straight left arm and a slightly bent right elbow) it will become much more natural.
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When James Durham recorded 94 at the Old Course at St Andrews in 1767, he set a course record that lasted 86 years. Golf: A curious sport whose object is to put a very small ball in a very small hole with implements ill desiged for the purpose - Sir Winston Churchill
Actually, both elbows were bent somewhat at address in my old setup. Now with the left straight at address the right is slightly bent. At half way up or so I find the slight tug with the right in opposition to the bending that should be putting me into the slot.
Will continue to try though. This move appears to require tremendous patience.