Hi Jorge,
Due to incompatibility of SD card image with eMMC first stage bootloader (this can happen with third party images), it may not be booting. Here are some things you could look at:
- To disable boot from eMMC, one can clear out area where the bootloader resides. This can be done manually or using this script. Be careful and confirm that the right device is being cleared.
- [for general debugging] I would recommend you to get a USB serial device. Helps a lot in case an image does not boot.
- [for info] Rockchip boot flow
Welcome to the ARM world! It takes some patience and hit and try in the first few days/weeks. Post here if something is not clear in above links.