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 missing headphones to provide an accurate calculation of how far away your device is.
Check the "last seen" time. If it's not updating at least every 30 seconds then you are too far away from your device, or the signal from your device is being blocked by another object. The good news is that your device is nearby. Keep moving around slowly so the app can receive more signals from your device.
You can also try shutting down our app and restarting your phone or tablet. Sometimes the phone's or tablet's Bluetooth gets "confused" and doesn't work properly.