Even when gals are totally turned on, and wanting lots and lots of sex, their bodies don’t always cooperate in the wetness department. This is especially true during marathon sex sessions. The best and quickest remedy for your situation is to use a great lubricant, like those offered by Astroglide. You may also want to spend more time teasing her during foreplay. Even though she may be up for the "wham, bam, thank you ma'am" style sex all the time, giving her body time to warm up and reach maximum response will help to get her juices flowing!
All the best,