Insufficient Financial Proof or 'Money Parking'
The bank statement does not show 500,000 THB (~$15,000 USD) or a recent single transfer betrays a deposit made just for the visa. Online banks without stamped statements are also an issue.
Solution : Maintain $15,000+ on your main checking account for at least 3 months. Request a signed and stamped statement from your bank branch. For online banks, get the PDF statement officially certified if possible.
~35% of refusals