On my iMac, in iTunes, I have one app (called "myApp", say) listed on Library > Apps. I want to transfer that app into my iPad. Under Devices > my iPad > Apps, I press "Install" next to myApp and the button label becomes "Will Install", so I expect it will be transferred to my iPad on the next sync process.
When I press the Sync button below, it starts to sync both devices, and right after it show on the status bar (next to the playback buttons) "Determining which apps to install" , I get the following message:
The iPad could not be synced because this computer is no longer authorized for purchased items that are on this iPad.
To authorize this computer for items purchased from the iTunes Store, choose Store > Authorize This Computer.
So I go to Store > Authorize This Computer, I type my Apple account password, next to my Apple ID, and iTunes gives me the following message:
This computer is already authorised.
Including this one, you have authorised 3 computers out of your available 5.
If at this point I try to sync again, I get the same message above saying the computer is no longer authorized.
By the way, on my iPad, under Settings > iTunes & App Store, the active Apple account it the same as on my iMac's iTunes.