Originally Posted by FlyerGoldII

As you are trying to get credit on on SQ FF plan - it is possible that your ff plan does not allow discounted economy fares to be credited, either on its own ticket, or on other Star Alliance frequent flyer partners' tickets.

I would suggest a couple of things:

i) mail a letter to SQ, and copy both UA and AC.
ii) if you do not get anywhere with option i), and if you already have another Star Alliance partner's account, simply credit your miles there.