Rear derailleur not shifting right/slipping

Hi guys, in the commute forum I was talking about this but I haven't been able to unravel it.

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However...I've got an issue with my rear derailleur. The low and high gears seem okay (9 cogs) but the 3-6 range is all out of whack, 4 especially. A lot of times, 4 just doesn't want to...