Same prob again-simply use-as a rough guide, times for next week-on same weekday. OR use bahn.de-of german rails and most wide planner as it covers not all, but most, and certainly for this. Most newbees tend to plan way too long advance-trains do not like that, as due to works, whatever there are always tiny changes.

Sometimes the prob may also be that you use names for towns in on lingo-but for Zagreb this should not form difficulties (though some stick to the old name of Agram-I bet youve never even heard of that)

A more reliable and up to date planner for international journeys is the DB planner. the rail planner app is one of the last places to look for reliable and up to date information.