When the apps stays in the "Walk Slowly" mode for a long period of time this means that it hasn't received enough signals from your lost Fitbit to provide an accurate calculation of how far away your Fitbit is.
Check the last seen time. If it's not updating at at least every 30 seconds then your are too far away from your Fitbit or the signal from your Fitbit is being blocked by another object. The good news is that your Fitbit is near by. Keep moving around slowly so the app can receive more signals from your Fitbit.
You can also try shutting down our app and restarting your phone or tablet. Sometimes the phone or tablet's Bluetooth gets "confused" and doesn't work properly.