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)