What can cause often occurance of caries (once in 6 months) despite following all hygiene and washing teeth regularly and abstain from sweets?

Recurrent Caries: Many patients think they are doing a great job of brushing and flossing when in fact they aren't. Some brush and floss well but BEFORE instead of AFTER eating; There might be an incipient cavity that is too small to be clinically seen or even apparent on an x-ray; A cracked filling right after your last check-up; Food impaction; Dry mouth from medication or medical condition; drinks high in acid.

Acid: Acid from bacteria residing in the nooks and crannies of your teeth, as well as acids from foods will demineralize the enamel matrix. You are asking for a guarantee: following good oral hygiene and diet habits = no caries. No such guarantee. Some pits and fissures cannot be effectively cleaned and will develop decay unless sealed. Do you floss? Are these recurent caries? Your dentist knows.