first: do you use the mix of traffic_models, that is predefined in LTE_trafficmodel.m? The delay is actually only evaluated for the delay-sensitive traffic-models voip, gaming and video (see UE.m, process_packet_parts).
If yes: Please check if in +traffic_models/generic_tm.m the function remove_packet is ever called. Here, the delay is written in the delay buffer. If it is always empty, it might be that for some reason you actually never finish the transmission of any packets.