The Thai Baht (THB) amount is calculated based on the BTC amount in your wallet.
Thus, it is normal for the THB amount to change according to the exchange rate which changes all the time. You can see the exchange rate at top-left on https://coins.co.th/. Please note that your balance will be calculated with the 'sell rate'.
What remains constant is the BTC amount in your wallet.
If you notice any unexpected changes in the amount of BTC, please review your transaction history (go to coins.co.th wallet) to review the incoming and outgoing BTC transfers initiated from your wallet.
If you have any questions, please reach us at:
Email: support@coins.co.th
Phone: +66 2 038 5038