My wife has a bicycle like this. It is only a few months old and it gets light use on weekends on (smooth) some dirt roads.
Recently it sometimes fails to downshift at the rear (i.e. shift to the smaller gears). When she stops I can see that the cable is loose. The spring in the derailleur doesn't seem to pull it tight. Moving the shifter or gently moving the derailleur fixes it temporarily, but then it may happen again an hour later.
What could have triggered the problem?
What should I try to fix it? Simply lubricate parts?