Andrew R Stewart 

Hanger alignment has become more important as the fudge dimensions have been reduced by the want of more rear cogs in the same old space. ( But I still feel it's mentioned far too much as a prime cause of shifting issues).

The very first thing I thought of is cable condition. Increasing cable friction initially causes problems at the small cog end of the cog set. That's where the der's parallelogram spring is at it's weakest and won't pull back as much cable if the cable friction gets too much. Make sure the cable's first inch is intact and not starting to fray/break. This is out of sight and inside the control lever. Detaching the cable from a frame stop or from the anchor bolt will let you push out that bit of cable from the control lever to inspect. Also the casing condition counts. That last bit of casing looping from the frame to the der is often the most friction prone, being the tightest curve and easily kinked somewhat right at the der adjuster.

The next thing I would look at is the upper/guide pulley. When new and in good condition it will have virtually no rocking slop, no ability to cock this way or the other way on it's bushing, it will have a side to side "end play". When the bushing is worn out it will tilt either way. This vastly increases the pulley's inability to guide the chain as the control lever wants it to. With the down shift over movement of the lever these shifts can be compensated for to a degree (just swing the lever a tad more, most people don't even know they are doing this all the time). But on upshifts the lever only allows the cage to more by the "when in good condition/new" amount. If the pulley is cocking the chain won't be "told" to go directly under that next smaller cog. Almost but sometimes not quite enough.

There are other factors at play but IME these two are biggies. Andy