You should look for your Endpoint FIFO configuration.
If your EP2 is Quad buffered and the interface is configured for slave fifo configuration and if clock is running on the interface,
then you should be able to make 4 successful OUT transfers.
If all the above are set correctly and even if you are seeing failures on the OUT endpoint transfers, please share your firmware.
Also let us know where did you obtain slave_fifo.hex file?