The problem can be any number of things.
When Alexa hears your command to turn something off, it sends that command to Amazon, which then connects to the skill of the light bulb (Kasa), which then the Kasa server connects to your light bulb and tell it to turn off.
As you can see, if any one of these steps encounters a problem, then the command fails. On Alexa's side, all it knows is to send that command. The confirmation beep only confirms that Alexa heard you and sent that command to Amazon's server. It does not verify that the skill is currently working, the Kasa server received the command from the skill, the Kasa server connected to the light bulb, or the Kasa server successfully told the light bulb to turn off.
Although it has not been confirmed, but the word on the street is that TP-Link is planning to discontinue the Kasa line of products. In the past when other companies began to sunset their products (e.g. Blackloud Smart Switch, Logitech Harmony) users have reported intermittent problems. That is, things that used to work fine started not working once in a while. And then that once in a while becomes more and more often.