Spire should automatically sync to your phone when it's within Bluetooth range whether the app is running in the foreground or in the background. If your Spire is not syncing, try toggling Bluetooth OFF and back ON through the Settings screen on your mobile device. You may also need to try closing and restarting the app or rebooting your phone.
On Android devices, you may also need to take an extra step by clearing your Bluetooth cache. To do this you will need to go to the Settings section on your phone and then go to Applications or the Application Manager. Look for the Bluetooth or Bluetooth Share app. If you don't see these apps listed you may need to tap on the 3 dots at the top right of the screen and then select System Apps. This should repopulate the Apps displayed. You may also need to swipe to the left so that you're looking at all the apps downloaded to the phone vs a view that only shows those apps which are currently running. While in the Bluetooth or Bluetooth Share app tap on "Storage" and choose the Clear Bluetooth Cache/Data button. After this has been cleared, try running the Spire app once more.