If no HTTP errors are being returned, but you are not getting the expected result, review any Conversation API callbacks you have received for more information. These can provide insight into more downstream obstacles.
Ensure that the server you are targeting in your API calls is in the same region as the one for which your Conversation API app is configured.
Review the corresponding channel documentation for any channel-specific information that may be interfering with your solution.
Reach out to your account manager for general information and to determine whether there are any issues with Sinch's system.