If the node is using ABP: this packet has FCnt = 0. Did the node somehow reset? The packet is ignored if the frame counter is not higher than the last known value. See ABP node stopped working -> Fixed by creating new ABP.. Why? :) - #7 by arjanvanb
As for network problems, maybe the following helps?
Beware that someone in that topic writes that you might need to re-register devices to the application.