Web Block not Rendering a Site
Some web sites explicitly disallows "embeddeding" in other content, which is what Blocks needs to do (i.e., embed it as part of a larger Blocks presentation).
This is disallowed by a website by specifying the "X-Frame-Options: SAMEORIGIN" header in its reponse.
Depending on whether you have control over the browser used by Blocks, you may be able to circumvent this using an extension such as this one:
This extension can be used as a work-around for this problem. Note that it works on Chrome only.