When a front derailleur won't shift to a smaller ring, it's often that the cable/housing needs to be replaced. LPH gave good advice about separating the effect of the housing from the effect of the derailleur itself. There are also small adjustment in the derailleur that could affects its shifting, eg, the stop screws and the cage angle and height. Shifters can also get sticky and stop working properly, so squirt in some lube, or soak in solvent and relube.