This lightweight mod shuffles all items every time an ItemEntity is created. What this means is that all mob drops and block drops will give you what you least expect. It also means if you break a chest, its contents are randomized, and chickens no longer lay eggs. The shuffling is per-world, so if you aren't having good luck, just press the big red nuke button and try a new world.
I hope you find some shulker boxes!