What is the right shoulder doing? Is it tracing the TSP on the downswing or is it inside of that (down the line view)? If it is inside of that then you might need to make sure you don't create too much secondary axis tilt at the top of the backswing or during the downswing.
Matt
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"In my experience, if you stay with the essentials you WILL build a repeatable swing undoubtedly. If you can master the Imperatives you have a champion" (Vikram).
The reason you can't sustain the lag is because you are so eager to make the club move fast (a reaction to the intent of "hitting it far"). So on a full shot you throw it away too early, which doesn't happen for your short chip. (bts)