Why won't my Pi boot from certain brands of SD card?

Are you using a beta Pi 4 EEPROM release (that's typically one with a pretty pink and white screen that prompts you to hold down shift for a network install)?

If so, install a stable EEPROM release (that's typically one with boring white text on a black background with no network install option) instead, because we've found that certain beta EEPROM releases won't boot TLXOS from certain SD cards, e.g. 16 GB SanDisk, for no good reason (i.e. a bug).  This problem has nothing to do with ThinLinX.

Assuming that you have at least one SD card that your Pi will boot from, you can use the Raspberry Pi Imager application (available from https://www.raspberrypi.com/software/) to upgrade/downgrade your EEPROM to the latest stable release of the Pi firmware by selecting "Misc Utility Images -> Bootloader -> SD Card Boot" (or use "Network Boot" instead, if you intend to use TLXOS' RPi's PXE boot feature) in the "CHOOSE OS" menu.  Write that image to an SD card that you know will work, then plug the card into your Pi and power it on.  After the screen turns green (to indicate a successful update), you can then try switching to your formerly non-booting TLXOS SD card to see if it now works.