There are a certain amount of things that can spawn in one given area. Let's say the number is 12, each of the 12 have a chance to spawn as either a chest or material. Basically the idea is to farm thoroughly, pick up every material and open every chest always, don't skip and run around like a ding ding because when you race back to an area on your sparrow you are interrupting spawn patterns. So make a farming plan and stick to it by running the same pattern every time and picking everything up and your chances of getting good items increases. Just a theory but it seems to work for me.