I am having problems with the 7 speed Shimano rear derailleur on a Raleigh Siren in that it will not shift into the largest cog on the cassette. It moves smoothly between the other six cogs on all three of the front chain rings. Before I start messing around adjusting screws etc can anyone on here give me some guidance please? I am attaching a photo of the derailleur in case that helps. This is the original derailleur, the bike being around 20 years old.