Struggling to flash a custom firmware on an AR300M-16-ext

Struggling to flash a custom firmware to a AR300m-16-ext. The flash is designed for a AR300M-16. On flashing the ar300m-16-ext goes into uboot recovery mode. I have popped the cover off the devices to check the boards for differences but all i can see is the non ext model is board rev v1.4.0 while the ext model is v1.5.2. The flash is not compiled by me but is available online.

The board rev version may not affect your firmware.

What is the problem now?