File sharing on wireless connection

I have an older IMAC and new Macbook Pro that I'm trying to get to work with my Airport Extreme. I have an old G5 connected to the Airport via ethernet and both the IMAC and Macbook can share files with that computer but when connected wirelessly they cannot share files with each other. When I connect the IMAC with an ethernet cable - they connect ok. All computers can connect with the internet with no problem wirelessly. The IMAC and Macbook have MAÇOS 10.6, the G5 has 10.5. File sharing is turned on on both computers, firewall is turn off. When I ping the internal IP address assigned to the wireless connection I get response "no host found", when I ping the ethernet connect I get good response. Please help, I would appreciate any suggestions.