Constipation may present when droppings are pasty and stick to the vent area and the vent may be clogged from inside. Using a very soft Q-tip/swab moistened with warm water, gently remove the droppings from around the vent. Once you have a clear view of the vent, using a new swab with a dab of KY jelly or a bit of regular cooking oil if need be, carefully wipe at and slightly into the vent (not actually inserting the swab).
If droppings don't begin to fall normally after this, placing the bird in a shallow pan of warm water may help get the chick going.
When this continues after these measures - see your vet without fail.
If you ever notice a ‘bubble' or what looks likes intestines or a red mass from the vent area you need veterinary intervention quickly.
You've pulled at the right time so it's good to know you're doing this right. Please be ready to get the chicks seen if necessary.
The American Cockatiel Society http://www.acstiels.com/Articles/BasicCare/cockatie.htm
The Bird Care Library at Harmony Animal Hospital http://www.petvets.com/petcare/birds.html#nutrition
Good luck!
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author