Denied access to enter Hong Kong

Hi, my name is Edward katongole. Am from Uganda. I visited HK around October And stayed for 30 days And later applied for extension and I was granted 1 day. Then on the 1st of November I traveled to mainland China and stayed 10 days there. But coming back to HK through Shenzhen border I was denied access to enter by the immigration and was sent back to mainland. Don't really know why. Please can you explain to me why I was sent back and what can I do to visit again without such issues. thank you