DataStores not updating, any help?

I'm trying to get my DataStores to work, but there's a catch...

In the first server, when I use

local DataStore = game:GetService("DataStoreService"):GetDataStore("Departments")
local key = "user_" .. game.Players.TheHospitalDev.userId
print(DataStore:GetAsync(key))

It will print "N/A", which is correct.

I then change the value. It prints whatever the value was, correct.

I then rejoin the server, try to print it again. And it prints "N/A", not correct.

My script to change

local DataStore = game:GetService("DataStoreService"):GetDataStore("Departments")
local key = "user_" .. script.Parent.userId

script.Parent:WaitForChild("leaderstats").Department.Changed:connect(function()
	DataStore:UpdateAsync(key, function(department)
		local newValue = script.Parent.Parent.leaderstats.Department.Value
		return newValue
	end)
end)

You need to wait until the player's data is ready.

Player:WaitForDataReady()