Chris hinted at it -- to get the cracks, you need to have the top set early, so it's not going to expand while the middle's still trying to rise, and there's still enough leavening left to break through the crust that formed.
You have a few things that can go wrong:
The oven isn't hot enough.
(it needs to set the top before the middle's fully risen) Not enough leavening (it needs to be strong enough to crack the top once it's set) Using a single-acting baking powder (double acting gives extra rise when it gets heated)
Not creaming the fat long enough
(creaming cuts little air pockets into the fat, which means the leavening has to do less work)
Making too thin of cookies
(you need enough mass in the middle
so it finishes cooking after the top's set).
Mixing the dough too much (you don't want to develop gluten; it needs to be tender enough to crack easily)
As Jennifer mentioned -- old baking soda can be a problem.
I'd also consider investing in an oven thermometer to make sure that it's really providing the temperature the dial claims, as those are the two things that are going to be the most difficult to judge if they're a problem or not.
You'll also want to make sure the oven is properly pre-heated (so it doesn't loose all its heat when you open the door to put the cookies in), and avoid opening the door again 'til it's time to pull them.
If you have to rotate them (as the oven cooks unevenly), do it as quickly as possible so the oven's not open for very long.
cooking.stackexchange.com/users/67/Joe
If you have a fan assisted oven you might try turning on the fan for a short period when the cookies have flattened out in order to form a crust as quickly as possible.
If a crust forms before leavening is complete, you should get cracks.
A tip that I have seen in another recipe is to pull each dough ball apart with your fingers into rough halves and then squash them back together, again roughly so that the join between the two halves is still visible.
This uneven clump of dough is more likely to produce cracks.
In my experience, however, the cracks are not as attractive as those in the linked image.
cooking.stackexchange.com/users/1549/Chris Steinbach
Use a couple drops of water on top of each cookie: Roll them into balls Place them on your baking sheet Sprinkle water on the top of each ball Put in pre-heated oven
cooking.stackexchange.com/users/14939/brad
Have you considered getting a new box of baking soda?
Or perhaps try using butter instead of the margarine?
Possibly you are not getting enough leavening to cause the cookies to crack as they bake.
The kind of fat used might also affect the way the cookies rise, not making them crack.
cooking.stackexchange.com/users/7060/Jennifer S
The best way to get crinkles on the top is to take the cookies out of the oven when cooked.
While they are still soft, use a fork and flatten them slightly.
Perfect crinkles every time.
cooking.stackexchange.com/users/41367/Elizabeth mountain
I do not refrigerate the dough for a long period but DO sprinkle 3-4 drops of water on the rolled, dipped in sugar cookies just before baking.
They always have attractive cracks on top.
cooking.stackexchange.com/users/44687/Judy
This works for me.
Half way through cooking pull out the cookie sheet and give it a good bang on the counter and then again at the end of the cooking time do it again.
Cracks!
cooking.stackexchange.com/users/70938/E. Mountain
I used  this recipe  to make ginger molasses cookies. Though the cookies tasted great, my cookies didn't develop any cracks in them like the ones in the picture. I tried baking them for longer, but that just resulted in them becoming more crispy and browned a little more.  How should I modify the recipe to get the traditional cracks in the cookie?  EDIT: The only modification I made to the recipe was to use unsalted butter instead of margarine.
How to get cracks in my ginger molasses cookies
cooking.stackexchange.com
<baking><cookies>
cooking.stackexchange.com/questions/25879
cooking.stackexchange.com/users/9262/Dharini Chandrasekaran
Make sure the oven is hot enough, you cream the fat enough and the cookies are thick enough. Adding a coule of drops of water to each rolled cookie before baking before placing in the pre-heated oven can help, as can using a fan assisted oven. Try replacing your baking powder if it is old and banging the cookie sheet on the counter half way through and after cooking. You should try increasing the heat of your oven, creaming the fat for longer and rolling out thicker cookies. You could also put a few drops of water on each cookie before cooking.
Check if your oven is hot enough. You might also be rolling out your cookies too thin or not creaming the fat for long enough. Use the fan on your oven to form the crust as quickly as possible. Sprinkle a few drops of water on the balls of dough. Your baking soda might be too old. Bang the baking sheet on the counter half way through cooking time.