Why won't my lcd after replacement light up or sometimes go white?

i am fixing a customers ipod touch 4g cracked screen and i have put in the lcd cable and the screen will not light up or sometimes it does but it is just white, please help me understand why this is happening. when i fix other ipods it does it as well and i just have to try taking out the connector and then putting it back in and the out again and then in, i just do that for hours and it works, but i am having no luck with this one can someone tell me why its not working or why they do this, thanks

Andre