In case Brink's suggestion doesn't work, you should leave the Homegroup from all machines in your network then create a new one. This will clear out the Homegroup credential cache.
Homegroups tends to be a little spotty through wireless connections but it can usually be made to work.
Create a new Homegroup then try rejoining this new Homegroup from the laptop through a wired
connection, after you are joined up go back to using wireless and check to see if you can still access the other Homegroup machines.
This procedure will usually fix Homegroup wireless connection problems.