What are the x-frame-options headers returning on your parent domain? You can inspect headers in the browser F12 console.
Per the docs
SAMEORIGINThe page can only be displayed in a frame on the same origin as the page itself. The spec leaves it up to browser vendors to decide whether this option applies to the top level, the parent, or the whole chain, although it is argued that the option is not very useful unless all ancestors are also in the same origin.