@weasel_atty 

A lttile know fact.  Routers do have connected device limits on each of their bands.  For instance on TP-Link routers these limits are typically a maximum of 32 or 64 devices per band.  Since you have 29 smart devices alone on your 2.4GHz network this is likely the issue you are experiancing.  One thing you can try is enable the 2.4GHz guest network and add the new smart plugs to that band, or moving some dual band devices to the 5GHz connection.

@Carl Thanks for the information. I added in a new access point and everything is working.

@weasel_atty I had the exact same issue. Switching the Linksys router security mode on my 2.4 GHz band from the default "WPA/WPA2 Mixed Personal" to "WPA Personal" solves the issue. It's weird, since the model 105 plugs I had previously never had a problem. 

weasel_atty wrote

I have four HS103 plugs, and none will connect to my wifi. Note that I also have four Kasa bulbs, five HS105 plugs, and 20 HS200 switches, all of which are working fine. The HS103s previously worked. Today they all were dead; no light at all. They can be switched on and off manually using the button on the device.

 

I reset them by holding the power button down for 10 seconds, and they go into orange-blue blink mode. I can connect to their wifi with my phone, and the app then asks which router they should connect to. I tell them and they begin blinking blue, presumably trying to connect. They never successfully connect. Instead, they time out and go back to blinking orange-blue.

 

Any ideas? Thanks.