Please give the details, date, time and route of the reservations that you have problems with and you will get advice here.

Please specify for each of those trains: origin, destination, date, departure time.

Only with that information, it will be possible to give any advice.

Prague - Krakow 2nd of August (any time)

Krakow - Vienna 5th of August (any time)

Ljubljana - Venice 17th of August (any time)

Ideally not very early trains or very late.

Krakow-Vienna you can book via tickets.oebb.at via seat only option. 3€/train

Ljubljana-Venice should be without reservation possible. If you choose the EC train to Trieste you need to pay about 3€ fee for the Italian part, also on board of the train or online via tickets.oebb.at - ONE WAY TICKET and at 1x adult select the discount Interrail/Eurail Global Pass. From Trieste take the first RV (Regionale Veloce) that runs to Venice (no reservation needed). 

Prague-Krakow it should work via cd.cz, at ÖBB it also works, but I’m not sure if it is ok for this train. 

For day trains:

Prague - Krakow: book via ČD, max. 60 days in advance.

Krakow - Vienna: book via ČD or ÖBB ("Seat only”), max. 60 days in advance.

Ljubljana - Venice: depends on the trains. No reservation needed until Trieste, in Italy you can take a regional train (reservation-free) or a long-distance train, which can be booked via ÖBB using these steps (do NOT use "Seat only”).

These trains usually have enough seats; there's no need to book multiple months in advance.