In the Star Wars movies, I always get the impression that the Sith/Empire/First Order spend a reasonable amount of their time looking for the Rebellion/Resistance (please say if i'm wrong), if so, then why do they find it so difficult? There are clearly charted maps of the galaxy which show areas under Rebellion/Resistance control, so why can't they just fly over and try and annihilate them?
There are no "areas under Rebellion/Resistance control." They don't control anyone except themselves; they live through a series of secret bases. They have friends, as mentioned in the Last Jedi, but these people are no more under their jurisdiction than the Empire themselves.
That said, the Empire/First Order does seem to be pretty good at finding these bases--the galaxy is big, there are a lot of planets from which to choose, and they seem to find a different base in every movie.