I've played with this. It seems like you can only make reservations for the present month, and to the end of the next month, for example, today, March 22, you can make reservations through April, but not May 1. So you might be able to make the reservation April 1st.
Are you sure the trains are full?
tiknet seems to give one the option, but I didn't take it through to the purchase, of course! Also Renfe has a
telephone booking number, where you can book regional trains you can't book on tiknet. Make a 3-way call with a Spanish speaking friend. I'd think they would be able to find at least a journey where you might have to change at Bobadilla, or Linares-Baeza. And actually, you can on tiknet book an 8:00 to Linares-Baeza, arrival 11:05, then an 11:45 connection to Granada. There's a 7:40 to Bobadilla (continues to Malaga), that arrives there at 9:15, then transfer to the 9:46 Andalucia Expres to Granada. So just explore all the options, and weigh travel time, cost, etc.