You enter a crime scene and discover one, single blood drop. You immediately measure the drop and calculate it’s Impact Angle. However, you will not be able to determine the Point of Origin of this single blood drop. Why can’t you determine the Point of Origin from a single blood drop?

You cannot determine the point of the origin of a single blood drop because it is just a single drop.

When a drop of blood splatters over the ground drag and trail occurs on it which could be useful when there are a lot of it and you can measure the distance from the origin but in case of a single drop it cannot help you because of one main reson which is distance.

You only CAN measure the direction but not the distance of the one single drop.

Let's take a scenario for a example.

A murderer approaches it's target from behind and grope it's target and stabbed him from front.

You will notice that the blood splatter does not occur in a straight direction but it spreads in kind of in a semicircle.

(THIS IS JUST AN EXAMPLE OF A SCENARIO. there are many other cases but this just an example to make the concept clear)

Hope this answer helped