I posted this at a different topic to which you posted the same problem. Please don’t do that. The quotes are a bit out of context:
I’m afraid you’ll need to capture a lot of raw uplinks to see if ADR is indeed set for all of them, and how the device is setting its ADRACKReq bit after not having seen any downlinks for more than ADR_ACK_LIMIT uplinks, being 64 in LoRaWAN 1.0.x (which might be a couple of days, depending on how often the devices transmit). Also, you’ll need to check if TTN is not sending any ADR downlinks at all, or when it is sending them: how those differ from that other network…
You might be lucky: when transmitting uplinks on SF12 TTN will force an ADR downlink even when not yet requested, to avoid the waste of air time.
Also, I assume it’s an OTAA device that last joined after October 2019?
What details did you get from the communication you had with them?