August is monsoon season and very few tourists, so these ferries - especially the tourist ones - do not run in the rainy season from June to October. Seat61 has the companies that do the trip. Viator is the most expensive one, book direct at the ferry comp., at your hotel. Or take a 5 hour bus. All very easy once in myanmar and hotels are happy to do a lot of booking for you, like flights, busses and ferries.
Due to lack of passengers, the boat companies do not run daily. Only with fixed departure date.
MGRG Express leaves on Aug 23. But Nmaihka leaves on Aug 19.
No boat run on Aug 20 so far.