The transactions are shown as “uncomfirmed” while they reside in the mempol, waiting to be included in the next block, and confirmed once they are actually included in a block. In Sia, a block is added to the blockchain every 10 minutes on average.
The inclusion of a transaction in a block can be also assessed by introducing the TxID (the hash) in a blockchain explorer, as:
While withdrawals from exchanges work in the same fashion as the rest of transactions, certain exchanges can experience delays in the transmission of a transaction. On May 2017, due to the unexpected increase of transactions volume (concurrent with a 20x coin value appreciation) the memory pool (mempool) of the Sia Client experienced problems that made some transactions to be dropped. After the issue was discovered, Sia developers provided tools to the exchanges to facilitate the re-broadcasting of these dropped transactions a few days after.
While issues with the mempool were completely addressed on version 1.3 (July 2017), on August 2017 users are still reporting considerable delays on Poloniex withdrawals. Even if the transaction is shown as “complete” on Poloniex, it doesn’t mean it has really been included in a block. Average delays are usually solved automatically in a few hours after Poloniex tries to re-broadcast the transaction. Users suffering the problem can check the provided “Txid” (the hash of the transaction) in one of the aforementioned block explorers to know if it has been successfully included in the blockchain or not. Delays longer than 1-2 days should be reported opening a ticket on Poloniex.