Why is there a slash through the https in my site's address bar sometimes?

Bassically it is what the question says, It is like my website it is not save for the clients. (I do not have online payments or somethig like that)

The thing is that this only happen sometimes, and it is not always in the same device, for example it could happen in my pc once a week and the rest of the time works just fine.

I have beeing reading and it could be a lot of causes, but in this way that it is sometime I thing it is really weird, my ssl expires in four day, but this problem has been happen for about two months (the site has been 1 year online).

When I call to external links (for javascript or css) I checked and it is always https.

I also read that could be the browser or the OS but this has happened in chrome, mozilla and IE as browser and Windows 10, windows 8, ubuntu (last version at this time), android 5.0.1 as SO and my web server is linux.

I do not know what other information it is relevant, I try to give all I read could be relevant in this case.