I have had the same outcome today. Poloniex had a message yesterday where they were having issues with STEEM and SBD, but the message cleared today, so I attempted to send a small amount of SBD from my Bittrex to Poloniex. 5 hours later, still not deposited.

I was able to find the transaction in the STEEM "blockchain" and it shows the transfer officially completed about 30 minutes ago, but still nothing in Polo.

Here is the chain site.

https://steemstats.com/#!/

I have been told that when making some transfers from Exchange to Exchange, you need to have a balance in the receiving account for it to work, but this still doesn't seem to be the case with SBD. I did see someone make a post about being successful today with STEEM, but nothing with SBD.