Not sure if this might be a tip on improving your chances in getting a successful SNA.

Recently I had a 3 night stay at a hotel that was almost fully booked due to a convention in the city.

Instead of booking a single 3 night stay, I made 3 separate bookings of 1 night each and applied a SNA to each booking.

At the end, for the first 1 night stay, my SNA cleared into a Junior Suite and for the second and third 1 night stays, my SNAs cleared into a One Bedroom Suite. I was able to ask the front desk to let me stay in the same suite for both 1 night stays and all I had to do was check out and check back in between stays.

Had I made reservation for a single 3 night stay, my SNAs probably wouldn't have cleared as the same upgraded suite needed to be available for all 3 nights (in this case, the Junior suite was available only 1 of the 3 nights and One Bedroom suite was available only 2 of the 3 nights).

When doing this, I was already expecting that there may be some nights where I won't be upgraded at all.

During the stay, I check marriott.com each night and the hotel was sold out in all room types each night.