There needs to be a delay of a couple of seconds between messages sent to the same chat channel or only the first message will be displayed with an error message appearing regarding the unsent messages. Latency can also affect the ability to send messages in succession.

If you're using a macro, you may want to check your formatting and make certain you're not double clicking the macro by accident. It can also be helpful to have a friend with you in the zone so if you get the error message, they can tell you what if anything appeared in the chat channel.