Total sessions and active sessions in threat-detection statistics do not indicate traffic drops. Dropped sessions should show up in fw-drop, insp-drop, null-ses and bad-acc, depending on the reason the session has been dropped.
In this sort of situations it is helpful doing a packet capture on the incoming and outgoing interface and compare the 2 and see if the ASA is actually dropping traffic.
You could also do a asp-drop capture for the specific IPs and see if the packets are dropped and why.
HTH
Bogdan