I have had SmartThings since 2014, I have about 30 devices from thermostats,motion sensors, lights, ect and the system I must say has been pretty rock solid UNTIL the past two weeks. Alexa decided she could not get a response from certain lights in my system and made it an absolute nightmare to deal with (when you spoil your family and then it doesn’t work, not good) I let SmartThings tech support know what was going on and they tried there best but could not help me. The issue is that when I would issue a command to Alexa to turn the bathroom lights on (two Sengled Bulbs), she would say the device is not responding and do nothing, if you issued the same on command immediately after she would do the command fine. I went to the log and saw the first command was simply causing a “refresh” command to be issued to the device which made it available for the 2nd time I tried the light. I found webCoRE and made a piston sending a refresh command to these problem lights every few minutes and my problem went away. I know its not a true fix as there is a problem with Alexa and SmartThings, however its a workaround if you are having these types of issues. Just thought I would share.
If your Having Alexa Troubles I figured out a fix
Thanks for posting this. I’m seeing the exact same thing. If I tell Alexa to turn on a light, she immediately says the device is not responding. If I immediately repeat the command, the device turns on. When I look at the IDE log, I see Alexa sent only a refresh command and immediately responded that the device wasn’t responding. I’m going to try your solution but would love it if someone could figure out why this is happening. It’s frustrating and makes family members think the entire setup is buggy, prone to failure and can’t be used reliably. I’d be interested in hearing if others are seeing the same thing. Maybe if enough of us chime in, the folks at ST and/or the Alexa support team will take an active interest and fix this.
I hear your pain 
Trust me my solution works, Alexa has been flawless since I put it in place last night. I know SmartThings is working on it however I think there is some finger pointing maybe between Amazon and SmartThings (just my thoughts). I just wish they would fix it so I didnt have to figure a fix out myself. I hope it works for you, it certainly put my house back to normal. If you have any questions let me know. Its possible you might need to play with the refresh interval a bit, I ended up needing to have it send the refresh command every 30 seconds to make it 100% functional again.
Kind Regards
Steve
The upper developers at ST know about the problem as the have escalated my continued complaining and they are emailing me daily to see if certain things they have done have corrected the issue. I hope as well that they get it figured out as it is very frustrating.