It sounds like you have done all the right things. I agree with Vettyy that you should use something like Screaming Frog to crawl all the old URLs just to double check there are no hanging 404 pages or missed http pages. Switching to 301 will take a few days to filter through, so you could run cache:domain.com in Google on your most important pages to monitor when they are being crawled. Also do you have a mix of http and https in Google in present? It may very well be something to just wait and monitor.
A good tool for sniffing URLs headers is Fiddler.

