Don't forget, though, that sales may be slightly down for a variety of reasons:
1) Election. Fans are saving their money for MNSG voting power, and Election rankings may well be more important than the sales for one summer single
2) It's the summer single. Generic song, generic PV, nothing as good or creative as U-19 was for the A-side. Fans have come to expect more polished performances from NMB and SKE, especially after U-19. Nagiichi, despite having Kpop levels of sync, is still a very AKB song and dance.

3) Exposure. I've barely seen any promotion for Nagiichi. U-19 was all over the place, and more importantly, many of the performances were in conjunction with AKB/SKE, exposing them to new people, not just the existing fanbase. There were even a few concert performances.