Line following robot using a 3 way line tacker

Hello everyone , i'm a beginner using Arduino , and i have a little problem powering my project .. I have two DC motors , a motor shield , a 3 way line tracker and an Arduino UNO board .. I've tried to power my robot with a nickel 7.2V/3000mAh battery by connecting it to the shield but the motors won't move .. i really don't understand why? and i'm pretty sure that it's not a code issue as tried each part alone ( the motors worked in a obstacles avoiding robot i've made before, the 3 way line tracker works with the same program 'that was meant for the line follower' only i used it with LEDs instead of motors etc ..) So plz could you tell me ,what kind of batteries do i need to make my line following robot move? And if u can detect any other issue i can't figure out plz let me know . Thanks all.