Why doesn't this function work more than one time?

works perfectly only the first time it happens, what's the problem?

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
 
local player = Players.LocalPlayer
local welcomePlayerEvent = ReplicatedStorage:WaitForChild("FishEvent")
local playerGui = player:WaitForChild("PlayerGui")


local function onWelcomePlayerFired()
do
local henlo = script.Parent.ActivateFish:Clone()
henlo.Parent = script.Parent
henlo.Name = "Clone"
henlo.Disabled = false
wait(4)
            end 
        end 

welcomePlayerEvent.OnClientEvent:Connect(onWelcomePlayerFired)

First let's get your variables down

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")

local player = Players.LocalPlayer
local welcomePlayerEvent = ReplicatedStorage:WaitForChild("FishEvent")
local playerGui = player:WaitForChild("PlayerGui")

The do on line 10 is pretty much useless so let's get rid of it, also lets combine the function and calling it.

welcomePlayerEvent.OnClientEvent:connect(function()

end)

Next i'm going to add your code in with just a bit of rearranging(idk i like it in a order ;p)

welcomePlayerEvent.OnClientEvent:connect(function()
	local henlo = script.Parent.ActivateFish:Clone()
	henlo.Name = "Clone"
	henlo.Parent = script.Parent
	henlo.Disabled = false
end)

No point in adding the wait(4) because the event can be called over and over

Let's put it together

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")

local player = Players.LocalPlayer
local welcomePlayerEvent = ReplicatedStorage:WaitForChild("FishEvent")
local playerGui = player:WaitForChild("PlayerGui")

welcomePlayerEvent.OnClientEvent:connect(function()
	local henlo = script.Parent.ActivateFish:Clone()
	henlo.Name = "Clone"
	henlo.Parent = script.Parent
	henlo.Disabled = false
end)

As long as no other script is deleting that ActivateFish instance you should be fine.