I wouldn't jump to conclusions yet about your detector being bad, although I would expect the charm to sound off easily with the ATX, chains aren't always easy to pick up.....even with a pinpointer. Pictured below is a chain (24") I have that I use to test all of my detectors, I've never had one that hit it well on an air test even the ATX. I just tested again to make certain I am remembering correctly and with the chain balled up, I do hear a slight tone change at about 1 1/2" at 12 sensitivity. With the chain tightly wrapped around my finger, I can pick it up just a bit further. This same chain I used to test the new Propointer AT and did pick it up but at only about 1", it has to be the design of the chain more so than the weight!
Now no way am I an expert with the ATX but I also test with a 14k hoop earring 2 grams with no response and a 10k 2.5 gram cross hits strongly at over 6".... a .8 gram gold tooth cap hits strongly at over 6".
I would do some more testing before I came to the conclusion that the detector is bad.
Cliff