I have the server set to easy difficulty and am on survival mode, yet hunger doesn't stay depleted. When I try jumping around and running, the bar goes down one but then instantly regenerates. If anyone can tell me why this is happening and how I can fix it, it would be much appreciated.
What may be happening is you are in a special area around spawn.
Many servers have an area around spawn where you cannot take damage or lose hunger. This is so new players to the server can explore spawn without the risk of killing themselves before they have even started.
Try moving out of the spawn area. You will generally be able to tell this as there may be markers on the terrain, or a message will appear in chat.
However, if this is your own server, I suggest you make sure it is not set to peaceful difficulty. In peaceful difficulty, your health regenerates instantly and you regenerate health quickly.