Probably a rate limiter as it's a public server, I believe using obelisk balancer you can specify more than one public server and it will balance the queries. Whether it does auto-failover or weighted balancing I don't know off the top of my head. Or you could go back to trying to install a local obelisk server, then you wouldn't need the public ones. I'm no expert, just trying to help, give the website/docs a read.