It probably has nothing to do with the Let’s Encrypt certificate nor your webservers TLS configuration, that’s all fine. See https://dev.ssllabs.com/ssltest/analyze.html?d=www.exifeed.com&hideResults=on&latest for an analysis of your HTTPS configuration: result = “A”.
What is not properly set up is your website itself: it contains a lot of non-HTTPS resources:
- 6 JPEG images;
- 1 PNG image.
A second run had 2 PNG’s and just 1 JPEG from a non-secure location. Seems to vary.
See https://www.whynopadlock.com/ and enter your website for more information.