Often, a problem is the result of some item in the web page; and something about that item and / or a web server that is the source for that item, is not secure.
Sometimes, when that is the case, a Reload of the web page might improve things.
Is an example, where sometimes a reload of the web page, causes the RED field with WHITE exclamation point, to change and return to being the expected padlock.
You’re not getting it. Would it really be that difficult to expand that “site is not fully secure” message to also include the reason it’s being generated?
Could you provide a link to the site? Also, do you have your Shield set to upgrade to HTTPS? If not, I’m wanting to assume that might be all it is. As usually those warnings pop up just because it’s on HTTP instead, meaning data isn’t being securely sent to and from that website.