One thing that no one has mentioned as a possible cause is the cable fraying inside the shifter. If your cables/housing have been on the bike for a while (depends on how much you shift and how clean the bike is...6mos > 2 or more years) the cable could be fatiguing and fraying inside the shifter. Check it asap to be safe, i just had one today and the shifter is ruined because the head came off the cable and is now stuck inside the shifter. Shift the bike up into the large cog, then stop pedaling and paddle the shifter back to the small cog position, but leave the chain on the large cog...creating slack in the cable. Undo the housing at the stop on the chainstay if you have external cables, and pull the housing away from the shifter. Push the cable out of the shifter while pulling in the brake lever so the end of the cable comes out of the shifter. Inspect the end of the cable for fraying. If it is frayed, obviously change it immediately. If it's not frayed but the cable stays "bent" a little, change it soon, or it will fray/break.