If you have a new 10 Stage Water Filter system:
The pin may need a little lubrication. Look down into the top of the diverter valve and place 1 or 2 drops of cooking oil into the tiny hole. Then very gently, pull the pin in and out a couple of times. Once you reattach the diverter valve to your faucet, the pin should fall back automatically when you turn off your faucet.
If you have had your 10 Stage Water Filter for a couple of months or years:
Most commonly, this could be due to mineral deposits.
Unscrew your diverter valve from your faucet and place it in vinegar and hot water (1:1 ratio). Soak it for 10 to 15 minutes. Afterwards, look down the diverter valve and locate a small hole. Add 1 to 2 drops of cooking oil into the hole and gently pull the pin in and out a couple of times. Reattach the diverter to your faucet and it should work fine.