Originally Posted by cny-bikeman

You sized the chain properly. Most likely the derailleur is twisted or the hanger is bent/twisted. Put the bike up in a stand. Put the chain on the 2nd largest chainwheel and the small rear cog. While looking from the rear of the derailleur toward the chainwheels the pulleys assembly should drop vertically from the small cog and neither lean in or out nor point to the left or right (rather than parallel) of the chainwheels. Pull back on the derailleur body so as to rotate the pulleys. They should stay in the same vertical/parallel orientation. A bent hanger can be straightened, a bent derailleur is much more problematic due to all of the pivots.

p.s. It is helpful if you post full information initially. If you had indicated initially how you sized the chain and what the problem was the first poster would not have wasted his time addressing the wrong issue.