New bikes out of the box, and bikes that have been ridden will require adjustment. There are two screws on the side of the derailleur that most likely need to be adjusted. If the H screw is adjusted properly the bike will shift flawlessly on and off the smallest cog. This is the screw that allows the derailleur to have enough "slack" so that the chain may move onto the largest cog. If it is slightly too tight it will be a little harder to shift, if it is entirely too tight, the chain will not be able to move onto the cog whatsoever.  Here is how to adjust the rear derailleur on a geared bike: