Storing Variables

A common mistake many developers make is they store a variable rather than grab the latest updated value. Here is an example using a BoolValue.

local boolValue = Instance.new('BoolValue', workspace)
boolValue.Value = true

local x = boolValue.Value
--since boolValue.Value is true, x is true

print(x)
print(bool.Value)

boolValue.Value = false
--what if we change the value?
--well, x is stored and will stay true
--we can check this interaction by printing

print(x)
print(bool.Value)

Output:

true

true

true

false

Correction

This same principle applies to your problem. You are storing Mouse.Hit.p as a variable when you actually want to get the updated version. An easy fix would be just to put mousepos inside your loop.

local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

while true do
    local mousepos = mouse.Hit.p
    print(mousepos)
    wait(1)
end