Pickup works equally regardless of the level of the Pokémon you’re defeating. It is a 10% chance per battle to get an item from pickup. These odds are comparable to if you did 10 battles, you’d likely get one item, or if you did 100 battles, you’d likely have around ten items. These are not absolute and completely determined by luck/RNG.
The possible items are affected depending on the level of your pickup Pokemon (Emerald onward).
You have just been unlucky.
Source + experience