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 or Changelly's current 48 hour limit.
3) Possible coin-specific delays.
4) Other network related reasons.
5) Maintenace of a specific coin pair by Coinomi or the exchange.
First, consult this article about the status of your shift and instructions for each case:
ShapeShift or Changelly will review your case and will refund your transactions upon confirmation of the failed shift.
If you couldn't identify your case in the article mentioned above, please open a ticket at ShapeShift Support or Changelly Support, and provide them with the transaction ID of the failed exchange, or the address you sent funds to.
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 "Overview" tab of the coin you exchanged - the list of all send and receive transactions is shown there, and that includes the exchange transactions.
To speed things up, provide them with a refund address for the coin that was exchanged.
Please note that Coinomi does not encourage address reuse.