The loot system is chance / luck based. When you mouse over an item or visit it's item page, the tool tip will reveal the drop chance of the item. This percentage is the percent chance of receiving the item based on a dice roll system.
So if an item is 1% chance of drop. This means you have a 1/100 chance of attaining the item when a kill occurs.
Analogy of how it works:
A ball is drawn at random from a box containing 10 red, 20 blue, 30 white balls and 1 black ball. The chance of drawing the black ball on your first draw is 1/61. You draw a ball, it is white. Before you get to draw again you must drop the white ball back in the box. Hence next time you draw, the chance of getting the black ball is once again 1/61!
This is how the game works. Hence even after you kill a boss 100 times, if the chance of getting an item is 1%, you can think of yourself as back in square 1. Sadly, that means luck is just not with you and perhaps you are better off trading for the item or buying it off the auction house.