3 room addition isnt getting much airflow. Help!!!!

We just added a 3 room addition. Two bedrooms that are 11x14 and a den that is 8x16. the problem is that the ducts i put in aren't cooling the area very well. I'm not sure of furnace type but i will get it. Each room i added a 6" flex duct from main trunk, which is in the basement, and ran them to basement wall which the lengths vary from 10' to 23'. Then i reduced to a 4" flex which runs in crawl space for the remainder of the distance, which varies 8' to 16'. They then run into a 4"x12" register on the floor. Why am i not getting any airflow to cool and heat these rooms? Any help would be much appreciated. Thanks