Bangkok Airways Flights Not Available from Phuket to Koh Samui

We are planning to travel from Phuket to Koh Samui on 25 Dec 2025 but cannot find any flights on Bangkok Airways website. If someone can help me out with following queries:

1. Is it coz the flights are full?
2. Is it, only online booking is not available on Bangkok Airways website?
3. Is booking from any airport office in Bangkok/ Thailand available?
4. What are the alternatives available? I ve searched for an option which sounds good but i would like to know the practical recommendations for Phuket to Koh Samui (Ferry Ride)?