When sending SMS, sometimes your messages will get marked as spam. If enough of these outgoing SMS get marked as spam, you will get put into a "cool down" period, a 2-hour block from sending any messages within the marked market.
You will know you're in a "cool down" period when a message pops up on your campaign send page, that looks similar to the below.
It is a temporary block that will automatically lift after 2 hours.