I actually watched multiple agents checking schedules and booking tickets for me on their reservation system.
They have one screen for schedule look up, and have to write down the a code to enter on the reservation screen.
Their system will not display any connecting trains, only direct if they exist. I wanted to go from Seville to Barcelona all in one day. Two direct trains Seville-BCN, both left later than I wanted. But there are hourly AVE trains Seville-Madrid, and Madrid-BCN. Since I knew this before hand, I asked the agent book the tickets that way. He had to manually input the information, looking up trains on each segment separately, and ticketing the reservation for each segment separately.
If I hadn't done my homework before arriving to make the reservation, I'd have been stuck with the direct train, since that was presented as the "only option" by the agent.
As for making reservations on trains wholly out of Spain, DB can do this, SNCF can do this, OBB can do this, but RENFE can't. She tried, looked the schedule up on DB's website (just like we do here), copied down the train numbers and tried, it just didn't work.
They're using a seriously old reservation system. Watch the agents struggle to book the reservation in Spain, and then watch the ease of the agents do something similar in Germany, France, and even Poland!
When I mentioned my difficulty making reservations in Spain to an SNCF agent in Paris, she told me that it is very diffiult for SNCF to make reservations/sell tickets for trips wholly inside Spain. The computers don't communicate.