There are several reasons this might be happening:
Wrong coin is send to the wrong wallet. Eg BSV is send for a BCH transaction.
Wrong or missing XEM message. While sending XEM, make sure that you've put the correct message. It's indicated here and looks like a combination of digits and letters.
In case you indicated or forgot to put the valid destination tag for XRP or the valid memo for EOS/Stellar, the transaction won't be processed automatically and you'll have to contact our support team in order for your transaction to be either pushed or refunded manually.
Other internal errors. Even our well-made system can experience lag or other internal issues. If you think this may be the case, please report it to [email protected]