I have already purchased Reservations on both the Bernina Express bus in Lugano and the scenic train leaving Tirano, but I have been unable to purchase the tickets required. Our trip is July 24, 2024.
Unable. Why? A few more details please. Did your credit card not work? Did you try to buy from RhB (the actual operator) or SBB (the Swiss National Railway)?
please elaborate the problem
You have till the morning of your travel to get your tickets. So you always have the option to just get them at the ticket window in Lugano, where you even can buy with cash. July is still ages away. Until recently you could not even buy tickets this far in advance. Nobody does this.
Depending on your other plans in Switzerland you may want to investigate a pass. A normal ticket Lugano - Chur via Tirano will cost you 108,- so you better either buy a saver day pass, or some other pass (how are you continuing after the Bernina Express?)
A Saver day pass for July 24 currently 52,- in second class, without any reductions. They will only get more expensive as your travel date approaches.
https://www.sbb.ch/en/buying/pages/kontingent/kontingent.xhtml