I am simulating a dense network in NS-3, with around 200 nodes each sending 1MB of data to a single AP/router. Although I have set the CCA threshold to a very low value (so that all 201 nodes are within each others' range, implying that hidden/exposed terminal problems don't occur), I can see that packet loss still occurs (data is not received from all the 200 nodes).
What could be the primary reason for this packet loss? Could collisions still occur in this scenario? One reason could be that 2 nodes stumble upon the same back-off counter, which leads to them transmitting at the same time. Are there any other reasons for collision or any other kind of packet loss?