Not getting enough power to drive a motor while using MOSFET
Without a schematic it is almost impossible to give a detailed answer. I still want to try. If the MOSFET works as a simple switch you should look in the data sheet for RDS On, if necessary you can connect an equivalent resistor in series and observe what the motor is doing. In general, I would also pay attention to the temperature that arises at the MOSFET. That would be a good indication whether the MOSFET is really fully conducting. A MOSFET that is only partially conductive becomes very hot.
The SI2300 has 20 mΩ RDS on which corresponds roughly to the resistance of a copper line of 1.5 mm2 and 1.8 m length.