1. Transaction hasn't been included to a blockchain. Cryptocurrencies aren't stable, so minor errors might occur. We can push a payment through if you provide the hash (a tx ID) of your transaction.
2. ETC and ETH confusion. The addresses of Ethereum (ETH) and Ethereum Classic (ETH) are of the same structure. If you send ETC or ETH, make sure that you've created an appropriate transaction on XCOEX. For example, if you create an ETH to BTC transaction, make sure that you send ETH, not ETC. Otherwise your transaction will be stuck.
3. Wrong XEM message. While sending XEM, make sure that you've put a correct message. It's indicated here and looks like a combination of digits and letters. Messages like 'Hey how are you', 'I love XCOEX' etc. are lovely but don't work, unfortunately :)
4. Other internal errors. Even our perfect system may experience internal issues. If you suppose that this is the case, report it to firstname.lastname@example.org