First, you can check the light status on the plugs. The Wi-Fi lights should be solid green.Otherwise you need setup them again.
Second, make sure kasa is logged in with your cloud account and remote control is enabled.
Or you can reset all the plugs and reconfigure them.

Did that - checked that. That’s the first things you check. This is beyond basic.

Did that - checked that. That’s the first things you check. This is beyond basic.

Move the plugs close to the router to have a check.
if reset and re installation can't help, better to contact support.usa@tp-link.com for help.
It is not a common issue.

That didn’t work either. Contacted support, they said to reset everything. I’m on the 5th device of 8, 2 have shown up, 3 are on their 4th go around on resets. After reset, I let them sit for an hour. I can connect to my Wi-Fi at my neighbors house so I know it’s not the signal. These have worked great for a year, hopefully this isn’t the beginning of the end for them.

You said you checked, but you didn't mention whether the devices were still connected. If they're not flashing they're still connected and the problem lies in the app, not the devices. Resetting the devices might be a workaround but it's not the problem (though a power cycle of them and the router can sometimes help).

Do you have a dual band network or any repeaters/extenders? If so, try forcing your phone to connect to the same band and ideally same part of the network as the switches. The discovery protocol doesn't work very well across network segments and it's possible your devices were assigned a new IP so are no longer reachable by the app.