I'm 24, and have been sexually active for about a year and a half now, with the same boyfriend. We are very open about our relationship, including sex. The only problem is that when he goes down on me, which is almost every time we have sex, it doesn't always give me pleasure, even if he does things that would, on another day, make me orgasm. It's like something's blocking it. I've noticed that it's on the days that I'm quite tired, or when I have something on my mind. I know that the psychological factor can make a huge difference with women, but is this normal? And is there anything I can do to change this? I'm not expecting to have an orgasm every single time and what I describe doesn't happen too often, but when it does, it's quite uncomfortable both for me and for my boyfriend, who realises that something's just not right.

It's so normal it practically defines normal.
Even MEN experience it, men who women often assume can have sex with anything that moves or doesn't move, anytime anywhere, which pretty much covers anything.
You know the corny stuff about soft lights and flower petals and music, sweet nothings and a glass of wine? That's all part of becoming relaxed after a distressing day. Save the quick heated passion for the times when you don't need to unwind.

Very normal.

Communciate with your partner and let him know that it is being tired and distracted and has nothing to do with what he is doing.

One of the big things is recognizing when you are stressed, tired, preoccupied, etc. and seeing what works for you and your partner to adjust to the situation. It may be at those times you skip oral or go about it in a different way.