As a lover of doing oral with my male friends, my question to you is : has your wife been in an accident or something that might make the act uncomfortable? Have you changed your diet? Do you reciprocate her favors? If she used to really enjoy it, there must be something that has flipped her opinion.
I cant say if its normal, and I certainly hope that 35 years from now I still enjoy blowjobs with friends (or husband if I ever have one).
I hear my mother and her friends joke at parties about sex and it not being the same as it was when they were in their 20s, and always hear their husbands say the same thing. Your situation might not be normal, but some reluctance to "go down" doesn't seem unusual.
Conversations about blowjobs are sometimes uncomfortable for guys to initiate, and I always ask a guy (especially a new friend ) if he'd mind if I suck on him. If its a new guy, after sucking for a while, I ask him if he'd like to cum in my mouth. I almost always initiate any blowjob activity.
Next time you get into that "maybe she'll give me a blowjob" situation and she refuses, ask her - NICELY - Why? What's wrong?
DON'T tell her she used to love (maybe she didnt but just did it because you liked it) - she'll know if she loved it or not. Ask her to maybe just stroke you for a while - tell her how much you like that attention .....
I don't really know - but I would think its a situation that must be handled carefully. Thinking about it as I write this, I remember a college girlfriend who used to give out a bj occasionally, and only because she knew her boyfriend really liked them.