screen doesn't forward keystrokes

I use GNU screen in Ubuntu Linux to connect over a RS232 serial port
to an embedded device (which runs Angstrom Linux) When I reboot my
embedded device I can see all the messages properly with screen, but I
cannot input anything on the target device.

I run the commands

$ screen /dev/ttyS1 115200

this connects me to the target device. The target is already booted up.
Now, if I input on the target via screen

$ reboot

I can see all the shutdown and booting messages, but I cannot input
anything via my keyboard. Eventually I arrive on the login screen of
my target device, but of course I cannot login. If I kill the screen
and then relaunch it with

 $ screen /dev/ttyS1 115200

everything is fine again. I have to access the bootloader shell on my
target device, so I need to be able to input things during the
bootprocess. Any ideas what is going on?

Thanks in advance

Lucas