Take a simple analogy: there is a circle and you measure a point on the circumference. Given that point, you now try to re-create the original circle. You can't as you do not have enough information.
OK - take 2 points: now you can create a number of circles with the only certainty begin that the centres lie on a line that passes the mid-point of the 2 measurement points.
You need 3 points to uniquely define a circle.
Relating this back to the original problem: you are measuring a magnetic (or other) field at one point. That tells you the magnitude of the field at that one point. However that tells you nothing about the strength of the source and how far away it is, nor in what direction. You need to also measure the direction of the field which gives you 2 pieces of information but still does not tell you all you need to know to recreate the source.
The bottom line is that you need to measure at more than one location to determine the source.
Susan