Sometimes when switches are not responding well, telling Alexa again to "discover devices" fixes things.
Wi-fi is not the same strength, even a few feet away, and changing location of switch, even by a few feet can help.
Wi-fi signals are not constant throughout the day or night; blame the atmospheric conditions. Checking signal strength with an app, shows this to be true. After all, it IS just a "thru the air" radio signal like for a radio or TV signal from antenna. Signal can be dropped. So, I have had to actually unplug the smart plug and back in to get it working right again because it had lost the signal.