The things that come to mind immediately are to 1) check the hosts file and make sure the new hostname has an IP assigned it, and 2) reboot. There are some things that are tied to the hostname in the network stack. You can find them and update them, but if your hostname is set up right with the hosts file as well, a reboot might be the thing. In the case you already have tried rebooting, still double check the hosts file. There isn’t much more to a hostname change as long as your pi has not been altered much.