Several factors can prevent you from seeing notifications from your iPhone on your Apple Watch. Most of them involve features not being on (for example, Bluetooth or the actual mirroring option) or being active. You might also have a general syncing problem between your devices.
If your Apple Watch isn't showing notifications from your iPhone, here are some fixes to try.
Make sure your Apple Watch is unlocked. The first thing you should check is your Apple Watch’s passcode isn’t active.