The Find My iPhone app (and Find My iPhone on icloud.com) will only list devices that are using the iCloud ID that you are signed in with. If your iPad 1 is using a different iCloud account than your other devices you will need to sign out, then sign in with the iCloud ID it is using in order to locate it. (It must also be turned on, be connected to wifi, and have Find My iPad and Location Services enabled in order to track it.)
This site contains user submitted content, comments and opinions and is for informational purposes only.
Apple disclaims any and all liability for the acts, omissions and conduct of any third parties in connection with or related to your use of the site.