[bitfolk] Renumbering IPs and Bind

I renumbered my Ubuntu VPS at the end of december, I had thought successfully, until this week I got an email letting me know that my website was still using the old IP address. I didn't disable the old address, but I did add the new one and change the address in the bind zone file. I updated the serial number in the file, which I thought was sufficient to send the file up the chain (pardon, my understanding of what's really happening in this step is sketchy), but nslookup confirms that the site is still using the old IP address. Is there something I'm missing to get the Bitfolk nameservers to take the updated file? Thank you, Mike