I have made NR derailleurs work on 28T rear clusters on numerous bikes. The trick is playing with the chain length and position of the wheel in the dropouts to ensure that the cage is rotated far enough forward so that the upper pulley of the derailleur is far enough below the 28T cog when in the next lowest gear. Chain length is absolutely critical to making this work. If you could post us a picture of the derailleur with the chain in this position we might be able to help.