Paid API returning empty text

Hi I am a new user of the paid subscription. I ran a few test yesterday and the first one went okay. After that all the translation return with empty text. The log on my app shows no issue, and the text was being translated. After task done I checked my files they are all blank.
I am using an app which take a folder of .txt files and do translation then export them as .txt file also. Can you let me know what went wrong? I was no way near the threshold I am sure.