The following cases have been identified so far:
1. You are trying to exchange a coin that Coinomi supports, but Shapeshift or Changelly don't. Unfortunately, our in-app exchange providers do not support all the coins that Coinomi supports. With 278 coins and tokens natively supported in Coinomi, this is understandable. If they provide support for that particular coin or token in the future, it will automatically be available for exchange in Coinomi. The list of ShapeShift supported coins can be found here (click on any of the coin icons of the exchange box to see the full list).
2. You are trying to exchange a coin that is supported both by Coinomi and Shapeshift/Changelly (see case 1 above), but that particular coin is momentarily offline for maintenance or upgrade, either by Coinomi or the exchanges. Please consult this list for up to date information on Shapeshift's unavailable coins.
We're hard at work adding more exchange options, and provide our users with the best rates available.