Why Do All My Visitors Go To My Old Server When I Already Changed DNS Nameservers

if you just made some changes to your domain, this will explain why its taking so long

dns propagation

dns propagation is the waiting time you have to wait until all the changes you made a spread out to all the routers in the world. usually this takes 48 hours or 2 days. but usually it doesnt take that long, sometimes is immediate.