Are you using File -> Upload via Programmer? If you load a sketch in this fashion, it will overwrite the bootloader and you will not be able to upload via UART. You will need to reburn the bootloader to reactivate this feature.
Are you using File -> Upload via Programmer? If you load a sketch in this fashion, it will overwrite the bootloader and you will not be able to upload via UART. You will need to reburn the bootloader to reactivate this feature.