How would one detect a mouse hovering over a billboard GUI?

Okay, this is the code we're working with. It's a local script placed inside a billboard GUI inside the head of a player's character.

-- variables --
Player = game.Players.LocalPlayer

Group = script.Parent
HitBox = Group.HitBox
Name = Group:FindFirstChild("Name")
Role = Group.Role

-- functions --
function DisplayUsername()
    RoleplayName = Name.Text
    Name.Text = Group.Parent.Parent.Name
end

function DisplayRoleplayName()
    Name.Text = RoleplayName
end

-- events --
HitBox.MouseEnter:connect(function()
    DisplayUsername()
end)

HitBox.MouseLeave:connect(function()
    DisplayRoleplayName()
end)

The issue is that for whatever reason, the HitBox (a textbutton) isn't picking up a mouse hovering over the GUI. I know this because I used breakpoints.

Yes, the script must be client-sided. I've already tried the same thing with a frame.

What am I doing wrong here?