Check that the plate/washer on the cable fixing bolt is orientated right. I did this once and spent hours trying to figure out what was going on since I had the same symptoms as you. Got out the hanger alignment tool and had a go. Turns out I had the little hook on the plate in the wrong spot. The bit of the angle washer that hangs over should be pointed towards the barrel adjuster. It should be perpendicular to the cable and not parallel, and not shielding it if you can see what I mean.
Getting this washer in the wrong spot will ensure that the body can't swing out enough to get down onto your smallest cog.
Check this link out:
http://techdocs.shimano.com/media/te...9830609123.pdf