Why does Humanoid.FloorMaterial only update when I jump?

it's only updating when you press space because you're only changing it when InputBegan fires

just move the dive cancel into the other InputBegan, then change the second to a GetPropertyChangedSignal for FloorMaterial

btw generally you shouldn't have to put several InputBegan connections in a single script, unless you need to disconnect one of them later

-- this should be in StarterCharacterScripts instead of StarterPlayerScripts since it relies a lot on the character
-- i don't actually know where it was parented but the lines of code that reset all the variables at the top made it seem like it was in StarterPlayerScripts :^))

local Player = game.Players.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait() -- no idea what the () after CharacterAdded was about
local Humanoid = Character.Humanoid
local Root = Character.HumanoidRootPart
local rs = game:GetService("RunService")

local diving = 2 -- global variables bad

game:GetService("UserInputService").InputBegan:connect(function(inputObject, gameProcessedEvent)
    if inputObject.KeyCode == Enum.KeyCode.LeftShift and diving == 0 and Humanoid.FloorMaterial == Enum.Material.Air then 
        print ("Dive!")
        Humanoid.MaxSlopeAngle = 5
        Humanoid.JumpPower = 0
        Humanoid.WalkSpeed = 50
        diving = 1
    elseif inputObject.KeyCode == Enum.KeyCode.Space and diving == 2 and Humanoid.FloorMaterial ~= Enum.Material.Air then
        print ("Canceled dive!")
        Humanoid.JumpPower = 60     
        Humanoid.MaxSlopeAngle = 50
        Humanoid.WalkSpeed = 30
        diving = 0
    end 
end)

Humanoid:GetPropertyChangedSignal("FloorMaterial"):Connect(function()
    if Humanoid.FloorMaterial ~= Enum.Material.Air and diving == 1 then
        print ("Hit ground!")
        Humanoid.WalkSpeed = 0
        Humanoid.JumpPower = 0
        diving = 2
    end
end)