Why does this part of a health script not work?

The reason it isn't getting to the if statement is because the script is still running the loop, and never checks the if statement.

Either way, the check would only run once. You need to connect both values to a .Changed event.

function onPlayerEnter(player)
    local playername = player.Name
    print(playername)
    local humanoid = game.Workspace:FindFirstChild(playername)

    local hunger = Instance.new("NumberValue") -- Creating the hunger value
    hunger.Name = "Hunger"
    hunger.Parent = player
    hunger.Value = 100

    local thirst = Instance.new("NumberValue") -- Creating the thirst value
    thirst.Name = "Thirst"
    thirst.Parent = player
    thirst.Value = 100

	function check()
		if hunger.Value < 1 or thirst.Value < 1 then -- Part that is not seeming to work
			while (hunger.Value < 1 or thirst.Value < 1) do -- Added this in, as I assume you don't want the health to decrease forever.
				wait(1)
				humanoid.Health = humanoid.Health - 1
			end
		end
	end

    while true do
        wait(0.1)
        hunger.Value = hunger.Value - 1
        thirst.Value = thirst.Value - 1
    end

game.Players.ChildAdded:connect(onPlayerEnter)
hunger.Changed:connect(check)
thirst.Changed:connect(check)

If this doesn't work, try separating them into two scripts; one for checking and one for decreasing. I think this should work, though. Hope this helped.