The link is to an article that will talk about why progesterone levels can be low. The article below gives some great suggestions to help increase her progesterone.

http://www.buzzle.com/articles/progesterone-deficiency.html

How to increase progesterone naturally?

Ensure you are getting enough micronutrients.
For optimal progesterone production the body requires adequate intake of B-Complex vitamins, in particular vitamin B6.
The other key nutrient in progesterone production is magnesium.
Eat plenty of organic dark green leafy vegetables, almonds, black strap molasses, seeds, nuts and beans. They are all good sources of B vitamins and magnesium.
Eat a serve of good quality organic protein, the size of you palm 3 times a day. Hormones are made of protein so you want to make sure you are getting plenty in your diet.
Chart your cycles, to see if your temperature rises in the second half of your cycle.
Avoid dairy and non organically grown meat and eggs. Animal products are the highest source of hormones in the human diet. One study found that a major source of animal derived estrogens in the human diet are milk and dairy products, which account for 60-70% of the estrogens consumed.

We usually associate dairy and drinking milk with calcium, and never think about what else we may be consuming along with the calcium (and dairy, by the way is not the best source of calcium). Here is a list of hormones which have been found in cows milk:
Prolactin, somatostatin, melatonin, oxytocin, growth hormone, lutenizing releasing hormone, thyroid stimulating hormone, estrogens, progesterone, insulin, corticosteroids and many more. Do you think an excess consumption of all these hormones could disrupt your own hormonal balance? You bet!
Most of these hormones are found in the animal, but many are also given to the cows by the farmers to make them grow big and fat, so that they get more money when they sell the meat. Charming!
So eat a diet abundant in organic fruit and vegetables, whole grains, beans, nuts and seeds and drink 2L of filtered or mineral water a day (I personally use and recommend the Aquasana drinking water system and this shower filter). Consult your naturopath if you wish to take supplements. Practitioner grade supplements are of higher quality then over the counter products and you’ll get better value and result for your money.
If you have low progesterone, increase your progesterone and increase your chance of a healthy pregnancy.