Try more than one offer before posting as it could be just that offer and it may have nothing to do with Copeac or any network for that matter. I have come across many offers across different networks that don't allow iframing.
I know for instance hydra blocks iframe's but if you add something like "&frame=keep" to the end of the URL it won't break the iframe. I am too lazy to look and see the actual tag I added. Ask your AM, sometimes there might be a trick to it.