Do you know the stall current of your motors at 12 V? Can your motor driver deliver this current?
One thing you need to note about the L293 is that there is a significant voltage drop between VIN and VOUT. If you look at the datasheet you can see that at 1 A the typical output low voltage is 1.2 V (max of 1.8 V) and the typical output high voltage is Vcc - 1.4 V (min of Vcc - 1.8 V). This means that on average your output voltage is 2.6 V less than your input voltage when you are sourcing or sinking 1 A, and worst case your output voltage is 3.6 V less than your input voltage. This would at least partially explain why you experience less torque when using the driver versus connecting the motor directly to power. I'm not exactly sure how this voltage drop scales with current, though.
One thing you could do is to use a motor driver that doesn't have such a large voltage drop, or you could increase your input voltage.
- Ben