Why is my iframe blank on my published website?

[halfinhalf]

[Note: The title of this post has been updated by a moderator.]

I'm trying to display a web page in an iframe using an embed element.

The iframe is displayed, but it's empty.

I know this can be a header problem, but I own the web page I'm trying to display, so I should be able to fix that.

When I view the headers for this page, X-Frame-Options isn't set but the content doesn't show.

When I set X-Frame-Options to ALLOW-FROM https://testingiframes.weebly.com/ - the content still doesn't show

When I set up an identical iframe on the same server as the content I'm trying to display, it works.

Do you have access to some logs that might help me figure out what's going on? 
(I'm developing content for a client and they need to display it as part of their weebly website - hence the testing)

Test: youtube displaying correctly, my iframe content not displaying

https://testingiframes.weebly.com/

iframe code:  

<iframe style='width:80%; height:2000px; border: 2px solid silver;' src='http://www.talkingspellings.uk/consulting/test.php'></iframe>

Thanks.

[NJRFTF · Accepted solution]

@halfinhalf

Inspector reveals your security issue as follows...

Your resource has to be loaded via https, so you'll want to edit your code so the URL used with your iframe uses https instead of http.

[halfinhalf]

That's very helpful, thank you!

[halfinhalf]

@NJRFTF That fixed it! Thanks again