Didn't they give you any contact numbers/e-mail contacts when you reported the missing luggage in KL? The feedback channel of SQ seems to be directly connected to a rubbish bin, normally not worth sending anything via that channel.

If you have status with them it might be worth to contact PPS-Connect or Krisflyer.

If you don't have above try to complain on their facebook/twitter accounts. Airlines tend to respond better there.