Talk to Gastroenterologist on Hepatitis B Vaccination

I have completed my engirex b course (0,1,6) in june 2019. Before going for vaccination I get the status of hepaptis b checked and that was negative. After that i started vaccination course and completed as mentioned above. Now after 6 months i have done anti hbs test and the value comes to be 6.33 which is less than the reports are attached. What should I do now?What can be be the reason that the value is less?Please guide and a lot.