Session data not being passed to iframe

So I'm loading some content on my site through an iframe on a different site and I'm session()->flash() to flash some data on the page. It works when I load the page but doesn't when I load it through the iframe.

I tried the P3P header workaround but to no avail. Can anyone share their thoughts on this?

The P3P header workaround I'm talking about can be seen in the answer on this SO question: http://stackoverflow.com/questions/8957769/php-session-iframe

I also tried adding it via .htacess on apache servers and on config files on nginx servers. Nothing seems to work. Please help.