In simply looking at the cover of Beatles for Sale, it is obvious that the band is worn out from touring and tired of being constantly hounded by the press, their fans, and basically everybody on the face of the earth at the time. This weariness really shows in the songs. Gone is the enthusiasm of A Hard Day's Night. Also, it appears Lennon/McCartney songwriting machine hit a snag when composing this album. "Eight Days a Week" is the only truly recognizable hit, though there are other great tunes on the album.
While Beatles For Sale appears to be an evolutionary step backward, it's important to remember that 1) there are still plenty of good songs on the album and 2) The Beatles moved forward from this point and created some of their best work.