Some women just don't get wet enough. It depends on how dry you are. If it's like a desert in there, you should talk to an ob/gyn. But if you just don't get an increase from a normal baseline, well, that can happen. Judicious use of water-based lube should fix it. Contrary to popular belief, older women are not the only ones who can have trouble with vaginal lubrication. Make sure you drink enough water, in case dehydration has something to do with it, but honestly this sounds like something that's pretty normal.
Hi Lx, You can try putting your self on a course of vitamin B6 I have an article about it the women who had the same problem as swears by it, she now only has a couple of days a month thats dry.
Or try this, google dodson and ross, I'm sure there must be something on there for you, if not you can Post Dr Betty Dodson, I'm sure she will find you a natural answer for it.
Good Luck
I am giving you a link which will lead to you a website which discusses the problem akin to yours.
http://www.reuniting.info/sex_perfection_marital_happiness/ch_5_six_rules_of_sex_intercourse_pt_4
I am sure the above information will solve your problem.As the article is longer one I cannot paste ii.