hey buddy , i think you just need to remind her that sex with you can be a wonderful , exciting thing and unexpected at sometimes .
for example , just go see her and pull out that salami you got (hopefully) and slap it a bit all over her and tell her "listen mami , remember all those times you had great moments with my sugar stick , yea? well what are you waiting for ? open up those sweet legs and let me in that love tunnel of yours !!! "
it is important to be very enthusiastic with this .
if this fails , bring her headaches pills , a fan and a blanket , at the middle of the day , then suggest to have sex , if she asks why you brought all that just say " well i brought the blanket so you can stay warm in case it would be too cold , the fan in case it would be too hot and the pills in case you suddenly got that "headaches" you seem to get often right before we are about to have sex , its the middle of the day honey (make sure to point out your watch) , not to early and not to late now heh , now let me at it !!!"
if this still fails just say something like "well what excuse are you going to pull out of your ass now" ... "wow iv had a wife for YEARS now and i cant even get laid by my own married wife !!! "
just something to make her feel bad and so she can finally spill it out , its good to communicate in relationships and sex is NOT an exception .