Arousal fluid is very light and evaporate easy. Your body also goes through cycles, during the day, during your menstrual cycle and during life when you produce less or more arousal fluid.
Other things that affect it:
* Sleep and stress
* Medicines including allergy, birth control, antidepressants etc.
* General health
How do yo fix it? Destress, sleep enough, lead a healthy life, exercise, cut back on caffien, alcohol and smoking, extend foreplay, make sure you are not dehydrated, do not panick or worry about it, it does not mean anything is wrong in your health, body or relationship. Use oral sex (cunnilingus) before intercourse.
A good personal lubricant makes all sex better. Water based is cheap and easy to wash off. It is lighter and you might need to top up during a long session. Silicone based lubes are very slippery and smooth, and lasts a long time.
You should also be doing kegel exercises to keep your pelvic floor in shape, regardless of your age. It is like a good lube, it makes sex better for you and your partner.