I apologize - didn't realize you said gold. SDC should be free. Should be able to use it multiple times. Of course, same fare class needs to be available - if not, you do have to pay the fare difference. However, I've never seen the web site or mobile app proactively offer changes to flights unless the same fare class was available - you usually need an agent to do that.
Does sound like a glitch - a call should do it.