How many rings does a normal call ring at someone's extension before it goes to voicemail? It should be MORE than 3 (if you want the call to ring at a phone 3 times, then go to the AA).
Try creating a new target line -- one that isn't associated with any phone.
Associate the line with a phone (try Ring Only & if that doesn't work, Appr&Ring).
Having the FNA or Fwd Busy ring count set to the same number of rings as the AA answering the line could cause issues.
Make the AA answer the target line after 1 ring (just for testing), and then up-it to 3.