Its just one of those things if something is set up with fine tolerance's, one day its ok then its not.
My rear mech had a disliking to a lot of hill climbing, if I did too much climbing, it became very difficult to change whilst in the lower gears and also start making a grating sound when in the lowest gear and if I tried to adjust them it made no difference or screwed up the higher gears. But I found that if I did not try adjusting them, they would be back to normal the next day.
Eventually I tightened up the B screw 3 turns, which stopped it happening, but slowed down the gear change, so I screwed the B screw in 1 turn at a time whilst out riding until I hit the sweet spot and not had problems since.