Sigma 30 and its Sticky Focus Ring

Hi all.  I have the Sigma 30 for a while, about 6 months now, and just got the Sigma 19.   In comparison the 19's focus is much smoother.  Lately I think the 30 is getting worse. Can't really manual focus with it any more as it sticks, and then as I apply more turning force it jumps.  This cannot be right.   Has anyone returned it under warranty for that reason?

Thanks

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