Some alpaca yarns don't hold shape well, so ribs will likely stretch out, cables will flatten a little, etc. For some things this is fine. It can still make a nice cuddly scarf, alpaca is extraordinarily warm too! but if you especially wanted the ribs to be obvious, then maybe you want to try a different type of yarn for the scarf?
I've seen examples of felted Alpaca and it looks great! Though if you can't see ribs now, chances are you won't see them much better after felting, but sometimes felting can pull things up more and make things (like ribs) show up better... :shrug: