no serial output and no booting with some kernel builds

I'm building my own kernel as I need some special drivers. For the
most part it works fine but sometimes booting on the same board with
the same U-boot and the same environment fails to boot. Replacing the
kernel makes it work again. The frustrating part is that there is no
data at all on the serial interface so it's hard to guess what goes on
as I don't have a JTAG interface.

I mean I see "uncompressing ....done"
and then absolutely nothing.

Did anyone see this happen? Any ideas?

That usually means either your kernel crashed before it could send
anything to the serial port, or the console tty device wasn't set up
correctly. The only solution is to examine what is different between
the working and non-working kernels and debug from there.