The most common reason your payment has not been rendered on your balance is that you use different Telegram account to access the bot than Telegram account you have used to log into tgdev.io website and pay invoice.
To see which account you use on tgdev.io website log into tgdev.io website and go to billing page. You will see a text like "Your balance is ... credits, username=..., id=...". Where username and id are details of Telegram account which has credits on its balance.
To see which account you use in Telegram bot send this message to the bot "/whoami". You will get a reply with username and id details of Telegram account which you use to access the bot.
You must use SAME Telegram accounts to access the bot and to pay invoice. If they are different, then you will not be able to use the bot.
IMPORTANT: PLEASE REPORT ABOUT FAILED PAYMENT ONLY AFTER YOU HAVE CHECKED YOU HAVE ZERO BALANCE ON BILLING PAGE.