How to control my robot?

Hi,

i have got these motors http://ram-e-shop.com/oscmax/catalog/product_info.php?cPath=218&products_id=2360 and i supply them with 4* 1.2 volt 2500mah nimh battries and they work perfectly

but the problem is whenever i try to connect them to the l293D motordriver to control via the arduino they like lose 30 percent of their total power they become too weak!

how can i solve this problem? whats the problem?