we have 5 sensors reporting to chirpstack. recently, our gateway was offline for roughly a week. later we noticed that two dragino temperature sensors reconnected successfully, while three dragino soil moisture sensors did not.
any idea what might explain the difference? is it related to hardware, or to sensor config?
This fully depends on the device firmware and configuration.
Technically, a device device does not need to "re-connect" after a gateway outage. E.g. it can keep sending data, it is just not received by any gateway thus these frames are lost. If the device does not re-join, after the gateway comes back up, uplinks are being reported by the LNS (ChirpStack).
Depending the device firmware and configuration, once every N uplinks, a device might validate if the LNS is still receiving data (confirmed uplink or through e.g. link-check mac-command). In case no downlink is received by the device, it might wait for some time before it tries to re-join. If the (re)join fails, it might use a backoff algorithm and thus it might take some more time before the device re-joins after a long time of failed (re)joins.
