Did you deposit straight STEEM from your wallet to Poloniex? Or did you convert it to BTC first? In the future I recommend converting first to BTC then depositing it to Poloniex as BTC and then buying STEEM on there. I have been doing that and have had no issues with BTC, but I have read quite a few posts where people have lost alt coins transferring, but no solutions yet. I hope you find one.