What format did you try to play? Did you enable the Extras repository (http://wiki.maemo.org/Extras) and install Extra Decoders Support? Is it any one of these formats: ".mp4, .avi, .wmv, .3gp; codecs: H.264, MPEG-4, Xvid, WMV, H.263."[1]

Be glad I didn't wake up on the wrong side of the bed today. If you want help, you'll need to be a bit more professional about it and be a bit more specific on what videos are not streaming. If something doesn't suit your needs, sell it and find an alternative that does.

[1] http://maemo.nokia.com/n900/specifications/