Sometimes, for a variety of reasons, there will be a spike in the number of BTC transactions that are waiting to be confirmed. That will cause a delay in confirmation times, and increases the price of fees required for a transaction to be included in a block. You can see the current number of unconfirmed transactions here: https://blockchain.info/charts/mempool-count?timespan=1week.
The transaction fees directly influence how long you will have to wait for transactions to confirm. If you choose a low value, it's possible that it can take many hours or even days for the transaction to confirm. Once it is broadcast, a transaction is stored by thousands of nodes all around the world, so it can't be canceled. Eventually it will either confirm, or it will be rejected and the coins will return to your address like it was never sent. Read more about cancelling or accelerating transactions here.
To make sure that your transaction confirms more quickly, you will need to send the transaction with an appropriate fee. In Coinomi you can manually choose how much you pay in fees. The transaction fees (default or custom) go to the miners of each coin and not Coinomi. There are 3 predefined settings for Bitcoin (default/normal/low). You are also free to enter any amount you wish in "... / Settings / Transaction fees", and apply use value by selecting the option "default" when sending.
You can get an idea of what fees to use with services like https://bitcoinfees.21.co/. If your Ethereum transaction is not confirming, you can read about transaction fees here to resend or learn how to replace a transaction here.
If you are receiving a transaction that still haven't confirmed, please contact the sender. Receivers are passive, meaning that the choice of fees should have been made correctly by the sender.
Note on EXCHANGES from BTC:
Please consider increasing your fees when performing exchanges via our in-app exchange providers, Shapeshift and Changelly. The timeout limits of those exchanges might be reached if the BTC transaction was sent with low fees and does not confirm within 48 hours. In this case, please open a ticket with Shapeshift Support at https://shapeshift.zendesk.com/hc/en-us/requests/new or Changelly support at firstname.lastname@example.org and provide them with your transaction ID and a BTC refund address of your choice. (Most usually your current Coinomi BTC "Receive" address).