What Causes Helium Balloons To Lose Their Lift After A Day Or Two?

The answer is pretty simple: the helium leaks out. Helium is a very small atom and latex is pretty porous at the scale of a helium atom (if you ever go looking to buy balloons, you will see that there are "helium-grade" balloons, which try to be thicker and less porous). The weight balance that keeps a balloon afloat does not leave a lot of room for leakage, so once a little leaks out the balloon falls.