Last time we took the bus we spent the night in Tulum. I was pretty surprised to find that there weren't any seats on the first class bus for the next day. The person at the bus station told us to just show up, we might get a seat. After looking at the schedule, we discovered there was a second class bus thad had plenty of open seats. Other than no movie and we had to put our own luggage on and off, it was a perfectly fine bus. It stopped more often, but it left earlier and we actually ended up in Chetumal ahead of schedule.
When I've looked at the bus schedule on line, I've never been able to firm up times until a day or so ahead. Try searching for a ticket for the next day or so and you may be able to buy one that way, just not far ahead.