Usually, it's from one violation or another of one of X's ad policies. Happens for the usual restricted content, flagged domains, heck it might even happen when you have too many rejected ads. Premium helps but even so, the ad system is strict. No way through that, there's ways around it though. If you're niche might run afoul with the policies, it might be best to consider cloaking and other approaches to the ad. If you already are, might be time to reassess your setup. All else, you could ring up the ads support and see if you can sort it out with them or ask for a manual review if none of the violations stuff apply to you.
Also also, double-check your ad content and links to be safe. A clean setup helps a lot in appeals. So I've been told, at least.
