If you've been successful at palpating before, and you can't now, I'd say she isn't pregnant. If you've never done it before, it may take a few litters to get the hang of it.
I usually hold the scruff with my left hand, with the doe's head turned toward me. It's easiest if she's on a carpeted area. I use my right hand to reach underneath, behind her ribcage. I poke upward gently but firmly into the abdomen with my thumb and forefinger, feeling for grape-sized lumps (at about 12 days). Don't be shy - you have to be fairly rough to hurt the doe. You should palpate between 10 and 14 days.
You're looking for soft (but firm), grape-sized lumps. You may also be able to feel droppings, but these are usually much smaller (think tiny blueberries) and are very hard and firm.
You can also VERY GENTLY palpate when the doe is close to her due date, at which point you would be feeling the hard skulls and the soft bodies of the kits. You have to be very careful not to hurt them at this point.
It can be hard to palpate. But give it a few tries, be patient, and you'll get the hang of it.