Wow. The stories avionics shops will tell pilots. LOL.
Receive level is called "sensitivity" and ability to not receive unwanted things is called "selectivity" and the two are polar opposites in RF engineering.
Generally if you raise one you lower the other.
Probably just aligned the thing per the book. Which really just entails injecting a weak signal with a signal generator and tuning pre-selector cavities so they're both properly selective at the desired frequency (pass) and band reject filtering of the relatively nearby FM broadcast band (reject).
I doubt they did anything to make the receiver itself more sensitive. Just adjusted the filters so they allowed the correct portion of the band through.
On the test gear, with the tuning misaligned, the result will be a weakly received signal, but not because the radio isn't sensitive enough. It's because the front-end filter is wiping out all the signal before it ever makes it to the rest of the receiver. A little twist of the non-conductive diddle stick, and boink... Big signal. And magically the other stuff isn't heard anymore either.
The more interesting question is why did it go out of alignment: Broken ferrite slug? Corrosion? Vibration? Capacitor drying out? Tin dendrites? Etc.
Kids. Sometimes they even forget what frequency excites water molecules and operates the microwave oven in the kitchen, and wonder if they've just found space aliens...
But really they're just watching Bob heat up a Hot Pocket for lunch...
http://www.abc.net.au/news/2015-05-...rcSite=news&WT.z_link=Change+to+standard+view