Before blaming the shifters, check your cable tension and derailluer adjustment, and even the mechainical state of the derailluers. A friend of mine, who hammers on her big ring all the time, couldn't get her front derailluer to downshift. She had neglected to oil the pivots and the derailluer had rusted from her sweat. A good shot of PB Blaster, chased with Tri-Flow, fixed it.