About transit VISA

Hi i came from Cathay pacific airlines from Beijing to Hong Kong and then Bangkok to Karachi. As being Pakistan national i thought that i will require a transit visa in Hong Kong but i did not require that because it was an airport transit and the next flight was within 3 hours as my travel agent told me too that's, its not required but after one week going back from Karachi air port the they said to me that i cant travel to Hong kong because i require transit visa. I told them that my stay is within 3 hours and then I have a flight to Beijing but they didn't agree because of this mess i had to leave my flight. Now can you please tell me what exactly happened it is a law in hong kong that you need to get transit visa from Karachi air port Pakistan and while travelling from Beijing to Pakistan its not required ?