When you use Cloudflare you must use their nameservers. They provide two. If their nameservers are down your website goes down. Therefore it is tempting to add a third and fourth nameserver. However, if you add other nameservesr (perhaps your original ones) as third and fourth nameservers at you registrar, these will be used some of the time, since they are picked randomly. Therefore some clients will be sent to your server directly. My solution is to add a third nameserver but not a foruth, creating some redundancy, and accepting that a third of the traffic will not go via Cloudflare.