I was also having issues using it. It worked smoothly for one trip but not for another. It was only after the price went up by ~$300 did it start working again (hmm, suspicious).
2 silly questions:
1) All flights booked via the Multi-city tool, when it decides to work, should be subject to the DSCs and not just the one-way/return flights?
2) Even though the number of SCs that show up prior to payment is the standard amount, the DSCs should come through as a separate transaction?
(This is the first time that I've purchased flights during a DSC sale - I'm trying to make sure that I'm not making any assumptions. I'm excited and am hoping for the best once the flights occur next year - I should have enough to almost get me to WP.)