It's not just leveling up nodes, you're forgetting the infusion part: we spend weapon parts plus a weapon that would give us weapon parts if dismantled. It's a double "loss" of weapon parts. Also considering that most of the engrams now dropping are armor engrams (at least in my experience almost 50%, if not more, are armor engrams), and that some engrams turn into motes or coins, and that Bungie nerfed weapon parts drops by 25%, [b]yes it's a common issue[/b].