Gum Problems - But I floss and brush - I don't get this?

My dentist states I am very close to having periodontal disease with my gums. I really don't understand because I maintain good dental hygiene (cleanings, fillings, etc.). I floss and brush, but she says I am not "flossing hard enough." ? How hard do you have to floss? I have sinus issues often and I am wondering if the bacteria in sinus drainage has anything to do with my gums. ??