The first you need to check is whether your transaction is showing as valid on a chain explorer like Etherscan.io. If you can see it as a valid transaction on the chain with more than 12 confirmations, open a support ticket pasting your Ether transaction ID in plain text -- please refrain from sending a screenshot as the tool we use needs us to copy and paste from plain text, not an image.
Why does this happen?
Ethereum is still in its infancy and in development. We use the latest software version but in some instances of an abnormal network load, or other events on the chain, some transactions may be hidden from our system. Once provided with your transaction ID we can easily locate you Ether in a matter of seconds and add them to your account.