I think this is a problem created by the (good) changes made to the game recently. Since it's so easy to level up your light and get to harder tasks you need to grind less. However with out that grind you don't build up the surplus of materials. I face a similar issue with my secondary characters with sapphire wire and hadronic essence. Since they went from 20 right to 26-27 I don't have a lot of those materials. So they need to grind through a few Strike playlists now and again. It's also hard as since I do mostly Raids and PoE and Iron banner right now I'm not getting much for engram drops.