Sometimes I can, sometimes I can't?

As some of you are probably aware, statistics show that only about 30 percent of woman are able to "get off" during intercourse alone without manual stimulation. Well, I happen to be in the 30 percent, granted that the right sex position is used. Here's the weird thing. I'll have no issue having orgasms during sex for weeks, and i'll do so consistently. Then, just like that, I won't be able to during intercourse no matter what position is used or how turned on I am. This will last for a week or two and then I'll go back to having orgasms during sex. I am having sex with only one partner. Isn't this weird? What might be the reason for this?