Why Does The Remote Event Fire For One Player?

##What is the problem?

I have a server. There is one player. Then, then, another player joins. The problem is that when I use the remote event, it only fires for the player that was already there, but, for the other player that joined, it doesn't work.

Server

--\\ Services
local repStorage = game:GetService("ReplicatedStorage")
--\\ Network Data
local remoteEvent = repStorage:WaitForChild("RemoteEvent")

remoteEvent:FireAllClients()

Client

--\\ Services
local repStorage = game:GetService("ReplicatedStorage")
--\\ Network Data
local remoteEvent = repStorage:WaitForChild("RemoteEvent")

remoteEvent.OnClientEvent:Connect(function()
	local gui = Instance.new("ScreenGui")
	gui.Parent = script.Parent -- assuming the parent of this client script is PlayerGui
	local txt = Instance.new("TextLabel")
	txt.Text = "Remote Fired"
	txt.Parent = gui
end)

You see, I am working with the Client(see the GUI's).

The problem here is your server; you made it fire all clients, but only once, so when the other player joins, the event doesn’t fire again and the player doesn’t receive the signal. To resolve this, use the PlayerAdded event of the Players, which fires everytime a player joins:

--\\ Services
local repStorage = game:GetService("ReplicatedStorage")
local Players = game:GetService("Players")
--\\ Network Data
local remoteEvent = repStorage:WaitForChild("RemoteEvent")

Players.PlayerAdded:Connect(function(plr)
     remoteEvent:FireClient(plr, nil)
end)

You also don’t need to use FireAllClients, just fire once to the player who just joined.

If it works for you, please upvote and accept this answer!