@emanent
Search for your TXID on https://siastats.info/navigator. If you can find your TXID there, the transaction is completed and the problem is with your wallet. If you can't find your TXID there, the transaction is not on the blockchain and you need to contact poloniex to solve your issue.
Make sure you are using the latest version of Sia, make sure your blockheight match what is shown on https://siastats.info/.
Do note that the latest version of Sia-UI (v1.3.3) contains a bug that will not show your transaction in the UI. To view all your transactions, go to the terminal in Sia-UI and type wallet transactions to view all your transaction.
All wallet addresses generated by Sia-UI will be linked to your seed, therefore all addresses generated can be used even if you forget to click save for those address. To view all addresses generated, go to the terminal in Sia-UI and type wallet addresses.