You can't find a thing when you don't know what you're looking for don't have the ability to see it. Whatever is up with us does not show up on standard tests, and the mechanism of malfunction is unknown. Perhaps it is due to a currently unknown process, one which happens within a cell or mitochondria, or in an interaction of nearby elements that does not involve placing something in one's bloodstream.
Imagine a civilization with no concept of electricity trying to troubleshoot a set of battery-powered flashlights. Some work, some are dim, and after a while of fooling with the dim ones to improve them, they go dead. Take one apart and it seems exactly like the good ones. The dead ones obviously must have been cursed, the solution which can't be disputed is to burn the bad lights to drive their demons out and find who is the local witch.
If you'd handed them a voltmeter and explained that it measures "battery magic", they'd be at step one, not knowing how to make electricity but at being somewhat able to measure it. But with no concept of electricity, much less voltage, they will never build such a meter.
* (for the analogy to work, disallow cutting open the batteries - they're vastly simpler than living cells).