This could happen with transactions involving your wallet, an exchange like Bittrex or Poloniex, or mining pools like Luxor or Siamining.
The first thing you should do is see if the transaction was added to the Sia blockchain. To do so, go to the SiaStats Explorer and paste the transaction ID into the search box in the upper right corner.
If your hash is not found in the database, then the transaction has not yet been added to the blockchain. it may need some time to confirm, or there may have been an issue with your transaction. Common issues include using an incorrect address or wallet downtime on the exchanges.
It can take over 10 minutes, and sometimes over an hour during peak times, for your transaction to be added to the blockchain.
If you are withdrawing from an exchange or a mining pool, and you do not see your transaction in the explorer within an hour or so, please file a support ticket with the exchange or pool.
If your transaction does appear in the explorer, that's a good sign. Your transaction should reach its destination shortly, but delays may still occur due to network or exchange issues.