How to have a proper collision detection in placement games?

The code I have so far doesn't work, I am thinking that it does not update fast enough. Here is the code:

local player = game:GetService("Players").LocalPlayer
local mouse = player:GetMouse() 

local grass = script.Blocks.GrassBlock
local grassClone = grass:Clone()
grassClone.Parent = game.Workspace
local grid = 2
local finalPos = CFrame.new()

mouse.Move:Connect(function()	
	grassClone.PrimaryPart = grassClone.Hitbox
	local floorX = math.floor(mouse.Hit.X / grid + 0.5) * grid
	local floorZ = math.floor(mouse.Hit.Z / grid + 0.5) * grid
	finalPos = CFrame.new(floorX, 3, floorZ)
	local checkCollision = grassClone.PrimaryPart:GetTouchingParts()
	
	for _, p in next, checkCollision do
		print(p.Name)
		if p.Name == "Box" then
			grassClone.PrimaryPart.BrickColor = BrickColor.Red()	
			grassClone.PrimaryPart.Transparency = 0.5
		else
			grassClone.PrimaryPart.BrickColor = BrickColor.Green()	
			grassClone.PrimaryPart.Transparency = 0.9
		end
	end	
	
	grassClone:SetPrimaryPartCFrame(finalPos)
end)

mouse.Button1Down:Connect(function()
	local grassClone = grass:Clone()
	grassClone.Parent = game.Workspace
	local checkCollision = grassClone.PrimaryPart:GetTouchingParts()
	grassClone:SetPrimaryPartCFrame(finalPos)
	grassClone.PrimaryPart.Name = "Box"	
end)

Thanks for the Help!