Is there any age difference? Have you become more physically fit? What is her physical condition? Post menopausal?
No offense for good suggestions more information is needed.
This is a very real complaint and I have the same one. My wife also isn't interested in non-intercourse sex (i.e., oral, hand jobs, etc).
I suspect this is why a large number of men look outside of the marriage for sex.
Hey bro. You can have your wife simply give you a handjob so she does not have to exsert as much energy to please you. Sounds like she doesn't mind but it takes too much energy out of her to give u want u want. As long as you don't make it a demand from her.