In the airport change your setting to WPA2 personal here if it is WPA/WPA2.
If that does not work.. then I would change both the wireless name (SSID in normal speak) and password..
It is easier to change name and password together.
Use short names with no spaces and pure alphanumerics. NOT APPLE NAMES. Those are recommended during setup but derive from a different network era. Jurassic.. when Apple ruled the earth. Now even apple uses SMB.. ie windows file sharing protocol.
Password same rules.. but as long as you need. Mixed case and numbers.. but stick to alphanumerics as it makes it easier to comply with SMB rules.
Sorry I need to add for guest as per your original post.
Many times people use no password on their guest account being less worried about security since guest cannot use local resources just internet. Are you sure you set the guest wireless password??