Why does my Posi-Temp shower, with waterdrop knob, only give me hot water or cold water and not a mixture?

In most cases, the knob has been installed upside down. To correct this simply:

1. Remove handle cap and the Phillips screw under the cap.
2. Pull the handle off the faucet.
3. Now replace the handle, only this time, make sure the point of the waterdrop handle is pointing downward to the six o'clock position.
4. Replace the Phillips head screw and handle cap.