You need to check daily specifically Wed @ 12p (local time). I do believe vendors have to return tickets they can't sell or get cancelled which opens up time slots. Also they have increased the number of people allowed to visit as of March 1 and each Wed they are opening up slots for the following week per their website... We went in Sept and I was able to purchase our tickets a few weeks prior by checking daily. You need to be persistent....it's worth it. Make sure you have the correct website and register account ahead of time. Good luck!
https://cenacolovinciano.vivaticket.it/index.php
The experimentation to gradually increase the number of visitors allowed inside the Museo del Cenacolo Vinciano continues: starting on Friday, March 1st, the number of visitors has been increased to 40 for all time slots, with an additional 105 daily tickets available (for a total of 215 additional tickets each day since the beginning of the experimentation).
Each Wednesday at 12 pm the additional tickets for the following week go on sale.
At this still experimental stage, it is essential to observe the utmost punctuality and adherence to the timetable and procedures for entry and visit. We therefore renew our request for cooperation from all visitors.