A problem with the key being a copy of a copy of a copy is a distinct possibility. With the keys that came with my truck, the run, start, off, and lock positions were pretty reliable, if a little fussy, but I could only get it into accessory about 10% of the time. Went to a dealership and had them cut a (Fortunately un-chipped) key from the VIN and it works just the way it's supposed to.
I'm sure it was your first reaction, but just in case, try the following while attempting to turn the key back and forth: Pushing the key into the ignition, pulling on it out just a hair, pushing the head to each side, jiggling the key back and forth, anything that might get the pins to line up just enough differently. A little 3-in-1 oil or similar into the cylinder might not hurt either.