Are we correct in assuming that your client is using LS .NET client library?
In that case, the "sendmessage" operation can only consist in a HTTP request to the Server, so we don't expect that it can be blocked.
Can you confirm that the block is in the path from the client to the server and not in some consequent data that are expected to go back to the client?
You can ascertain this by looking at the Server log, in which all the client requests are logged;
you should recognize a proper send_message.txt request and compare the time with the client-side action.
Just ensure that the LightstreamerLogger.requests logger is at INFO level.