Which dates? You can still visit a International ticket office at the station and try to reserve your trains there. Many Railcompanies are connected it´s not as good like 20 years ago. But sometimes you and even the Staff behind the desk can be surprised what is still possible to book.
Give dates and times and where you have tried to get the booking and someone here may be able to help.
Day trains in Romania are best booked at a Romanian ticket office. 1 day or a few days in advance is totally fine.
Further, it would be good to know in which countries you'll be before you get to Budapest (Austria or Germany by any chance?). That would make it easier.
Further, please provide dates and departure times so we don't have to guess.
18 July 2022 Budapest-Keleti - Timisoara Nord 22:45 overnight train
20 July 2022 Timisoara - Craiova 21:48 overnight train
31 July 2022 Split - Villach Hbf 08:31 - 22:49
@seewulf @Al_G @rvdborgt
I’m going from Vienna to Budapest.
You should be able to book the first train (Budapest - Timisoara) by phone via ÖBB (+43 5 1717) and pick up the reservation when you get to Austria. Also try to book the second train there - not sure if it's possible. If not, book it on arrival in Timisoara.
Split - Villach:
Split 8:31 - Zagreb 14:54: book when you get to Croatia. Price is only ~1€ but still mandatory.
Zagreb 18:38 - Villach 22:49: reservation is optional. If you want a reservation, book it in Croatia. This train has 1 coach (no. 305) that only runs up to Villach so that's a good one to use (the rest continues overnight to Zurich).