Upgrading access points (APs) from 802.11n to 802.11ac can offer higher throughput and improved performance. However, to realize these benefits, client devices must also support the newer standard. If the clients remain on 802.11n, they cannot take advantage of 802.11ac features such as wider channels, higher modulation schemes, and beamforming.
Therefore, the most likely reason for the unchanged throughput is that the clients were not upgraded to support 802.11ac.
Reference: CWDP-305 Official Study and Reference Guide, Chapter on Infrastructure Design

When APs are deployed in the same physical pattern across floors without channel and physical spacing considerations, co-channel interference (CCI) between floors occurs. This is particularly problematic when APs on different floors are vertically aligned on the same channels.
From CWDP-305:
"In multi-floor deployments, avoid placing APs on the same vertical axis across floors using the same channel. This leads to CCI. Stagger AP locations and vary channels to reduce interference."
- Reference: CWDP-305 Official Study Guide, Wireless Design Process Chapter