Most smart home devices use small, low-power radios that only support the 2.4 GHz band. That band travels further but is far more crowded, so these devices sit at the fragile end of the network by design.
They also reconnect poorly. When a router restarts or changes channel, a phone rejoins in seconds while a plug may wait minutes or need a power cycle.
Address changes cause the second wave of problems. If the router hands the device a new address and the app cached the old one, the device is online but unreachable from the app.
Distance is deceptive. A plug behind a metal appliance or inside a cabinet may show as connected during setup and fail permanently once the door is closed.