What are the sizes and age of the SD cards? Have you tried updating the X-T3 to the latest firmware to see if that fixes your problem? I believe the latest firmware is v.210 (FWUP0019.dat) for the X-T3. Copy the firmware, downloaded from the Fujifilm support site, into the old PNY card that works to update the camera. Let us know if that worked for you. Good luck.