Why do my welds only sometimes work properly?

Hello, so I'm welding a model to a character and having difficulty. The model I weld either works perfectly or sometimes, inexplicably, will just fail to weld properly. It will appear for half a second and then, unanchored, will fall through the world and destroy itself as if the weld is non existant.

After some testing I've narrowed it down to this; the weld is just not appearing in the model properly SOMETIMES and other times it works fine.

I've used 'repeat wait until() weld exists in the right place' to try and fix this but then it just waits infinitely. This is the script I use, I really hope someone can point out what I'm doing wrong to try and get this working 100% of the time.

local Event = game.ReplicatedStorage.AccessoryEvents:WaitForChild('HeadAccessory')
local function OnEventFired(player, detail)
	if detail == 'accessoryA' then
		local Accessory = game.ReplicatedStorage.Accessories.Head.Hat:Clone()
		local g = Accessory:Clone()
		g.Parent = player.Character
		local C = g:GetChildren()
		for i=1, #C do
			if C[i]:IsA("BasePart") then
				local W = Instance.new("Weld")
				W.Part0 = g.Middle
				W.Part1 = C[i]
				local CJ = CFrame.new(g.Middle.Position)
				local C0 = g.Middle.CFrame:inverse()*CJ
				local C1 = C[i].CFrame:inverse()*CJ
				W.C0 = C0
				W.C1 = C1
				W.Parent = g.Middle
			end
			local Y = Instance.new("Weld")
			Y.Part0 = player.Character.Head
			Y.Part1 = g.Middle
			Y.C0 = CFrame.new(0, 0, 0)
			Y.Parent = Y.Part0
			C[i].CanCollide = false
			C[i].Anchored = false
		end
	end
end
Event.OnServerEvent:Connect(OnEventFired)

I wonder if there is some property of welds I'm missing that means they can't fix properly while the character is moving sometimes or what, it breaks usually after the character has moved for the first time and is then standing still.

the weld between middle and head is made many times in the loop, I don't see how thats what you would want to do. also you clone twice. I dont see whats wrong though, does it still happen when you comment out the line Y.C0 = CF... ?

I'm guessing g.middle is a basepart and then in the loop you create a weld between middle and middle (itself). It shouldn't be a problem but I don't know what happens when you start setting C0 and C1.

btw why not weld the whole hat beforehand to the middle part, so you can take out the loop and only need to make 1 more weld and set one C0? in any case I would first try moving g.Parent = player.Character to the end (after anchored) to see if the problem persists

more unwanted advice: in general consider making a check when a client sends an event to your server (instead of just giving them some item)