Why a copy of a copy lets you down
Most spare keys get made from another spare. Each time that happens the cuts drift a little further from what the lock was built for, and after a few generations you end up with a key that only turns if you lift the door, jiggle it, or hold your breath. Nobody notices the drift until the one key that still works snaps off in the cylinder.
A good copy starts from a good key. When the original in your hand is the clean one, it gets copied. When every key in the drawer is tired, the better route is a key cut to the lock itself, so the new set matches the cylinder rather than the wear on someone's old spare.
There is a quick way to tell which kind of key you are holding. A sharp-edged original slides in and turns without effort; a tired copy needs a wiggle, catches halfway, or only works when the door is pushed. Bring the best one you have, and the technician will say whether it is worth copying.

