To get to the bottom of this it would be very helpful to get any output, any error messages at all. If you do not have a USB UART serial adapter, maybe now that you have the display working you can take a photo of what u-boot is complaining about in your case?
u-boot will try to find the following files in the first partition of usb, sd-card or mmc:
/boot/extlinux/extlinux.conf/extlinux/extlinux.conf/boot/boot.scr.uimg/boot/boot.scr/boot.scr.uimg/boot.scr
The file boot.scr is generated by the flash-kernel package and its template looks like this:
How exactly it should look like depends on the kernel you have installed because template variables like @@KERNEL_VERSION@@ have to be filled with the correct values. You can find a working boot.scr in the first partition of the system images you can download.