Grid Placement only Placing on one side?

Okay, so i have a script which highlights a block when you move and when you click it places the block.

It works just fine, but it only seems to work on 2 of the 4 sides of the square. The other two sides snap to the top of the block, but the other sides place blocks on the side easily. Here's my code.

grid = game.Workspace.Garage.Grid
player = game.Players.LocalPlayer

tag = Instance.new("StringValue")
tag.Name = "block"

local mouse = player:GetMouse()	

prevBlock = nil
mouse.Move:connect(function()

	if prevBlock ~= nil then
		prevBlock:Destroy()
		prevBlock = nil
	end	
	
	if mouse.Target ~= grid and not mouse.Target:FindFirstChild("block") then
		return
	end
	
	blockPrev = Instance.new("Part", workspace)

	blockPrev.BrickColor = BrickColor.new("Bright green")	
	
	blockPrev.Anchored = true
	
	prevBlock = blockPrev
	
	blockPrev.Size = Vector3.new(1,1,1)
	local pos = mouse.hit.p
	blockPrev.Position = Vector3.new(math.floor(pos.X)+.5,math.floor(pos.Y)+.5,math.floor(pos.Z)+.5)
end)

mouse.Button1Down:connect(function()	
	local block = Instance.new("Part", workspace)
	block.Anchored = true
	
	tag:Clone().Parent = block

	local placePos = blockPrev.Position

	blockPrev:Destroy()
	block.Size = Vector3.new(1,1,1)
	block.Position = placePos
end)