I’ve had this happen in the past.  You can check a few things.  Go to your Sent folder, and click the envelope name, then go to More and History.  There, you’ll see the date/time the email was sent.  Beyond that, you can reach out to DS Support and they can provide you a tracelog.  

I’ve also had users complain of this and then come to find out they did a declared signing order and the first signer hasn’t signed yet--so subsequent signers not yet getting the envelope was the expected behavior.