The escapement mechanism is the mechanism, working similarly to the escapement in a clock, spaces the carriage over to the left side of the typewriter (right side of the paper) one space at a time, either pressing with any typing key or the space bar.  There are a number of problems that can occur here:  The escapement wheel, since it has been almost forty years since its last maintenance session, could be gunked up with dirt, dust, and other grit--as well as the loose dog and rigid dog.  Here a good cleaning and light oil will probably suffice.  The next thing that can happen is that the universal bar frame (the curved piece of metal behind the type bars that's pushed by each type bar to engage the loose and rigid dogs to push the escapement forward, may be out of adjustment, as could be the space bar.  There could be an impingement (rub) between the carriage and outer shell slight enough to still enable the carriage to be moved by hand, but not the spring drum.  Also the spring drum might not be wound as tightly as it should be--or both.  It could be a combination of all of the above, since the machine, in addition of the very long time since its last lookat, might have been improperly stored, with, say, heavy boxes mounted atop with nary a care of what that delicate machine is down below.  After all, the last person probably didn't have too much regard to the machine, as it wasn't--say, a walletful of money or a Rembrandt picture that could be sold for plenty of cash, or something else more profitable and more immediately pleasurable to the masses than an ordinary typewriter could afford.