You say you got 'a replacement' but do not specify for what: a replacement for the 'middle click' sounds strange.
Two things can go wrong here.
# The middle click not working: If you open the mouse you'll find the wheel shaft rests on a small micro switch. Determine if it seats properly and if you can hear a 'click' sound when pressed.
# If the wheel doesn't scroll, here's what I found on mine: A plastic wheel with a soft plastic/rubber 'tyre' (or tire in American spelling) covering it. My tire was slipping on its wheel - both very smooth - and the tire wasn't a tight fit any longer. I roughened up the wheel a bit and dabbed a drop of super glue on the wheel bead then re-fit the tire - problem solved.