l293d is not able to drive two 300 rpm motors simultaneously
Check if both channels are working in either direction individually. 
Check the current draw under the load per motor using an Digital Multimeter as an Ammeter function.


The L293 is designed to provide bidirectional drive currents of up to 1A at voltages from 4.5V to 36V.
600mA per channel for L293D


As long as your motors at drawing more than 600mA per motor, it should work fine.