Sorry to hear this man!
Til now I've always used BlockTrades, the platform that my STEEM account linkes too. This one has always worked, but the only drawback is that it charges a lot in fees I think... according to research, I believe that BlockTrades charges between 3-4% in fees which is a lot if you think about higher deposit/withdrawal numbers...
Let me know if you suddenly was going to receive your SDB on Poloniex. Maybe transaction time is VERY delayed.