One odd thing is that you say you have synced 3 catalogues. You can only sync one. Do you mean collections?
As for signing in, initially you will have to be online, either through a SIM card or on a network, because you have to point the app to your account and let it sync photos. After then, when you're offline you don't need to sign in start using the app. You're not signing out, are you?
Thank you, I did mean collections.
Problem solved. I didn't think I had signed out on previous occasions but now I am able to restart the app without problems!
I was running a test to see what difference it made to enable offline editing for a collection. I am surprised to find the resolution of the enabled collection is little different to the collections I have not enabled. Very good for offline sharing on the iPad.
There's nothing more frustrating than being offline when you want to show some pictures - and finding they aren't available. So I enable offline editing for all collections - and check that the collection's counter (to the right of the collection poster frame) to confirm they are all downloaded.