Don't assume the chain is too short until you've gone through the simple procedure to determine the proper length. Typically remove the chain from the rear derailleur cage and the wrap it around the largest rear and front sprockets. Then add 2 links. This is for most if not all Shimano.... I don't know if that holds true for others.

There are certainly adjustments that can make the difference too, and if the bike at one time did go into those gears but now doesn't, it's probably adjustment.