IME the SPG website was never very good at identifying why SNAs couldn't be applied. Sometimes one had to call, for example when the hotel has suites in their SNA upgrade pool which aren't available for dates of the reservation (isn't the point of SNAs that they do not clear until a sweep happens at 5 days up to one day before arrival, so of course availability can change in the meantime?), but one should be very clear that the phone agent understands exactly which room/suite upgrade categories are being requested and doesn't just check all available boxes.

I've generally had good experiences with SNAs, having earned ten in each of all but the first year and having in total either one or zero expire, where in many cases the use of SNAs is denied at the end of the sweeps, but I still get good upgrades upon arrival for the stay. However, you do need to be a bit strategic and plan ahead, in your case noting that the remaining SNAs of your first set of five might be able to be combined with some of the next set that you'll earn either later this year at 75 nights or next year when you hit 50 and 75 nights. SNAs at worst can be an excuse for a short staycation to enjoy holiday lights and shopping at the end of the year. YMMV.