Nano compatibility problems

Hey guys, looking for a little help

For some reason my Nano is not compatible with my Monster Wireless FM transmitter that I have in my car as well as with a portable sound system made for the iPod. Both of these work with my 30gig Video iPod and I figured that since the docking port was the same for both iPods it should work. When I plug the nano into either of these devices all that happens is that the Nano is charged. But it will not play music. Is the Nano not compatible with other iPod devices or is this just a problem with mine?

Thanks in advance for any help.