marriage sexual health -what could be the problem?

I am a 42 yr old married to a 42 yr old husband who does not seem that interested in marital relations, once every week at the most once every 2 weeks. I am slim and am ballet teacherand work out at the gymm and in very good health, and still attractive to other guys.... i can still turn heads. what from a guys prospective could be the problem. i have checked computer and no porn has been found. our realtionship is decent and steady. we have been married for 17 yrs. I even tried to buy lacy silky night clothes..... he has not requested me wear that and its been over 2 months since I bought it..... help guys... any suggestions? he is shy and doesn't talk to other women.