why isn't my device showing up on find my iPhone

Because either:

• It’s powered off
• It doesn’t have a SIM card
• It doesn’t have an Internet connection
• It’s battery has run down
• It is in a location that is shielded
• Bluetooth on the phone is not turned on (used only for crowd-sourced locating)
• You didn’t enable Find My on the phone

If you go to https://icloud.com/find and sign in with the Apple ID and password that your phone is signed in with is the phone in the list of devices? If it is then Find My was enabled on the phone, otherwise it was not.