Better late than never? lol Well, here's my take, even though some of my point were already touched upon...
On occasion, I have this problem, too. Usually, it's when I'm mentally horny but physically wiped out and tired. Other times, it's as someone else mentioned, I get it into my mind that I have to cum and I don't. I especially used to have a really hard time reaching orgasm while having oral performed on me. I'd get really close, then I'd start worrying about if I was taking too long and whether or not my wife was getting tired or whatever and then forget about it...it would never happen. I'd wind up having to finish myself and I'd feel bad and she'd feel bad, thinking that she hadn't done that good a job. Not much fun, all the way around.
Here are a few suggestions that have really helped me, they may work for you, too...
Most importantly is communication. I had always sort of assumed that giving me a BJ was sort of a no-joy prospect for my wife. Talking about it, however, and being complely honest...well, I came to find out she really enjoys it. Sure, there are some times when she really doesn't feel up to doing it for an hour, but most of the time she doesn't mind taking her time and making me feel good. Knowing this has allowed me to relax and stop worrying, which in turn has allowed me to just let it happen and it's pretty rare when it takes even a fraction that long anymore.
Another thing I discovered was that she really gets turned on watching me masturbate. So on the occasions when, for whatever reason, I just can't seem to relax and cum, it's "okay" for me to finish myself off. She enjoys watching me do it, I get the release, and all is well with the world.
My wife and I have been together for almost 14 years. She knows me better than I know myself sometimes. Since we've learned to open up to each other and share our fantasies, she's proven to have an uncanny knack for saying the right thing at the right time to get me "over the hump". Talking dirty definately helps!
Lastly, I've finally come to a point in my life where I've realized that it's OK not to have an orgasm. For years, there were plenty of times when I'd cum and she wouldn't and she'd reassure me by saying that while cumming is nice, there is a lot of enjoyment to be had just from being together. I never really understood/believed that until recently. Now, if I'm really tired or I can't seem to relax, or the planets aren't aligned, or whatever the reason...I don't worry about not being able to cum and when she does, we just sort of curl up together and take a nap and try again later. Ironically, now that I've grasped the concept that it's okay if I don't cum, it almost never happens. lol Sort of a nice side effect, wouldn't you say?
Not sure if any of this helped in the least. Just thought I'd throw in my $.02 worth.
Cheers!
~Mike