According to this StackExchange post[1] it seems that your website is secure and the SSL is working as intended but you may have added mixed content such as page references or images that are not secure.
[1]https://security.stackexchange.com/questions/147928/what-does-this-https-not-fully-secure-warning-mean -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/4e6808b6-d582-46d4-a695-47f9fca86298%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
