System Preferences > Trackpad > Point & Click
Uncheck the box beside "Tap to click" and recheck it again.
If this does not help, reset PRAM and SMC.
1. Reset PRAM. http://support.apple.com/kb/ph14222
2. Reset SMC. http://support.apple.com/kb/HT3964
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".