Are you actually providing the proper control signals to the driver?
Are you pulling up the enable pin (or setting it HIGH through either digitalWrite/analogWrite if the enable pin on the H-bridge is connected to your arduino)? Unless the enable pin is HIGH there is no current flowing to your output pins.
Also maybe having a common ground between your motor supply and the arduino supply.
1.1. POWER OUTPUT STAGE The L298 integrates two power output stages (A ; B). The power output stage is a bridge configuration and its outputs can drive an inductive load in common or differenzial mode, depending on the state of the inputs. The current that flows through the load comes out from the bridge at the sense output : an external resistor (RSA ; RSB.) allows to detect the intensity of this current.