What is the frequency at which you are accessing the device s25fl208k? Maximum frequency supported by this device is 76MHz. Can you please reduce the SPI frequency and try to read manufacturer ID and device ID?
Eventually I connected a logic analyzer and realized that the CS line was going up and down every byte I sent to the memory, so the command was not executing, and the flash chip not responding.
Now I set CS line low for the whole command and it works.
Thanks for your support,