I currently have 8 (working on more) devices connected to the smartthings hub. When I have my Amazon Echo discover new devices, it consistently only finds 5. I’ve changed the name of the devices it can’t find, restarted both, and reconnected, but nothing has worked so far.
Assuming these are device types the echo can recognize, did you authorize the use of the devices through the SmartThings app? Each device has to be individually authorized before echo will be able to discover it. See step 10) in the FAQ.