Did you build your own h-bridge ckt, or is it a commercial board?
Did you read the datasheet on the L298 'before' starting to play with it?
Those devices have certain requirements for minimum logic and motor-input voltages. Also, because
they are linear analog devices [BJTs, and not MOSFETs], there is a 1-1.5V loss of voltage between the
battery input and the motor output.
Also, for reference, the Tamiya twin gearbox motors don't draw a lot of current, and can be run off
of 754410 and L293 chips = 16pin DIP.