Equinox

I had a full dura ace set up and I was getting sloppy shifts in the middle of my cassette. My LBS said that I may have worn out a couple of my favorite cogs. It made sense so I repaced my 12- 27 with an 11-28 Ultegra cassette. I made some minor indexing adjustments and adjusted the B-angle to clear the 28 tooth cog. It still was very hesitant shifting from 14 to 13. All the other shifts are acceptably crisp. They repaced the cable, but that one shift is very iffy. Why would that one shift be a problem?

flip

Could be a bent derailleur hanger.

Equinox

How does that happen? I have not dropped the bike.

Retro Grouch 

I would definitely check the hanger alignment, if for no other reason, just to rule it out as a possible cause of your problem. It doesn't have to be very far off to affect shifting on 9 and 10-speed cassettes. Doesn't take much of a bump to tweak it.

FBinNY 

You didn't say, did you replace the chain?

I ask for two reasons.

1- you never replace a cassette without replacing the chain, since it's a worn chain that causes the cassette wear. Under normal circumstances you might replace 3 or more chains for every cassette.

2- a worn might have been your problem in the first place. As chains wear they not only stretch* but get more flexible sideways. This can make shift more sluggish especially on smaller sprockets where the jockey wheel is farther away. With the added flex it takes more horizontal deflection to cause the chain to move over and make a shift.

BTW- regardless of whether the bike fell over, double check for a straight hanger, like Flip and Retro Grouch suggest, otherwise you could spend lots of time and dough chasing your tail.

* Chains don't stretch, pin wear allows the links to move apart slightly.

Equinox

When I took the rear wheel off, I noticed that there was play in the cassette. The cassette was installed correctly and the lockring was torqued to specifications. It turns out that the freehub body was loose and needed to be tightened.