None of the other Star Wars movies have ever followed on directly from where the last ones left off. This is basically what the chronology of the films look like:
Episode II: 10 years after Episode I. Anakin has grown into a man.
Episode III: 3 years after Episode II. The events of the Clone Wars happened.
Episode IV: 19 years after Episode III. The Skywalker twins are now young adults.
Episode V: 3 years after Episode IV. Vader's been searching for Luke while the Rebels have built a base on Hoth.
Episode VI: 1 year after Episode V. Luke has completed his own Jedi training.
Somehow the Empire has managed to construct a second Death Star in the last four years despite the first one taking over 20 years to build. 
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Episode VII: 30 years after Episode VI. Some major stuff's happened.

Episode VIII: ??? year(s) after Episode VII.
Episode IX: ??? year(s) after Episode VIII.
So we really don't know how much time may have lapsed between the events of Episodes VII and VIII. And a lot of stuff can happen in between. Keeps those Expanded Universe writers employed.