Perform these 3 steps first
1) Check your SiaPrime version in the ‘About’ tab of the UI. The ‘SiaPrime UI version’ should match the ‘SiaPrime version’, and both should be the latest version which you can check on https://siaprime.net/#download. Additionally, confirm your wallet is synchronized by running the command ‘consensus’ in the ‘Terminal’ tab and ensuring ‘Height’ matches the most recent block on https://siaprimestats.info/navigator.
2) Double check the correct address. If this is the problem and you were sending to an exchange, contact the exchange. There’s a slim chance they may be able to recover the funds, particularly if the address accidentally sent to was an old exchange one. Otherwise, the coins are unfortunately most likely irretrievable.
3) Use the transaction id (txid) in a block explorer, such as https://siaprimestats.info/navigator. If no transaction is found, then the block containing the transaction has not yet been mined; wait until the next block is mined. If the transaction has been found, then it has been finalized on the network. If sending from an exchange that does not provide the txid, contact them.
Sending to an exchange
As long as the transaction shows up in the block explorer (and transaction reads confirmed in the UI wallet) then the exchange has received the coins, and will often require 6 confirmations (approx. 60 minutes) to become usable. If it does not show up, contact the exchange for support.
Receiving to your SiaPrime wallet
If your wallet is synchronized and transaction is not showing, then try a force rescan.