one of the possible problem would be your n900 hardware revision. see:

http://talk.maemo.org/showthread.php?t=40136

To find your hardware revision:

cat /proc/cpuinfo

So if you have a 1801, thread above, post 26 tells this:

http://talk.maemo.org/showpost.php?p...4&postcount=26

"I just flash my pr-production phone sucessfully just now, so pls follow my steps to flash yours:
1. Turn the phone into flash mode, don't ask me how
2. Why flasher can't flash the image in your phone The answer is the HW Revision, you HW revision is lower than what the image requires, so we need to change the HW Revision first. Using this order: "flasher.exe --set-hw-revision=2101" (Of course you can change the 4-digital number to the latest one), then plug out the usb cable, turn off the phone by removing the battery
3. Turn the phone into flash mode again, and now we'll flash the OS into the phone, using this order: "flasher.exe -f IMAGE Name -F" and the flasher is working on writing the image to the phone, it'll take about 2~3 minutes, and then restart your phone manually (I don't add the "-R" for it'll show a mistake when flashing the cmt-mcusw at the first time)
4. Turn on your phone and enjoy your new life with N900!"

edit: Because i finally found out how to link a post