Downshifting problems 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?
Once possibility is a rusty cable/housing.  When the cable or cable housing gets rusty it can be the case that the spring of the derailleur can't provide enough force to overcome the friction of the cable moving through the housing.  You can test this by disconnecting the cable from the derailleur and seeing if it is difficult it is to pull the cable manually (after you've switched the gear down).  If this is the problem, the solution for this is to replace the gear cable.