To explain my theory: Imagine the force is like a pendulum.
Pre Phantom Menace: Force in balance for like a thousand years
TPM: The force in flux, starting to lean to the darkside.
The jedi get caught out by the Sith, as the bias of the force starts to affect the Jedi.
AOTC: The lean continues. So much so now, that Yoda and Mace consider telling the senate that the lean to their ability to use the force has diminished. The Jedi now seem pretty blind to the massive sith plot under their nose..
ROTS: The swing to the dark side is fully done. The Jedi are screwed, all the force advantages have been with the sith, Yoda can do bugger all and goes into hiding.
18 years of darksidey galaxy history later, the force seems to start to lighten up.
ANH: The 1st flicker of light as Obiwan begins training Luke.
Alderaan is destroyed then so too is the deathstar.
ESB: As the shift back to the light side continues Palpatine is less powerful and cannot see Vader's plot to overthrow him with Luke etc
ROTJ: Light side bias wins out when Palps is so blinded that he didnt see the old reactor shaft trick coming and Galactic peace is restored.
The pendulum then settles somewhere in the middle and balance is bought about.