You might be using workgroup on one and public file sharing on another. Make sure you are using one or the other on both machines. This could also be a wireless networking problem. Sometimes just rebooting or refreshing will make the other machine appear.
Can the wireless machine see the other if it's plugged in with an eithernet cable?
Since you are running two Windows 7 machines you can also try using the Homegroup network which works pretty well for me. Homegroup - Add Computer or Join
To change sharing settings in Homegroup. Homegroup - Change File and Folder Sharing Settings