Sometimes an exchange may fail, for a variety of reasons. Most commonly:
1) The amount sent was over the variable minimum and maximum limits set by ShapeShift or Changelly.
2) The transaction was sent with low fees, resulting in long confirmation times, over ShapeShift's current 48 hour limit.
3) Possible coin-specific delays.
4) Other network related reasons.
5) Maintenance of a specific coin pair by Coinomi or the exchange.
First, consult this article about the status of your exchange and instructions for each case: HOW TO - Check the status of an exchange I performed in Coinomi
If you identified any problem using the guide above, please open a ticket at ShapeShift Support or Changelly Support, and provide them with the transaction ID of the failed exchange (in text form), or the address you sent funds to. In case of Changelly, also provide an address of the coin you sent, where you wish to receive a refund. Our exchange partners will review your case and will refund your transactions upon confirmation of the failed exchange.
Note: If your exchange history is not visible for any reason, or the failed exchange is not showing up on that list, you will find the transaction in the main tab of the coin you exchanged - the list of all send and receive transactions is shown there, and that includes the exchange transactions.