problem when charging via computer

i bought a broken ipod on ebay planning to fix it up and sell it for more. i replaced the mother board and battery with brand new ones and restored the ipod. it seems to work fine, but when i connect to the computer it seems like the battery gets drained. it makes a connection and i can put songs on it, but it doesnt charge it. the thing that confuses me though is that if i charge it with a wall charger, it charges fine. i know its not the motherboard or battery thats the problem because those are new. help please. thanks.