Power Line Communications standards, such as Homeplug and IEEE 1901, aim to provide strict channel access prioritisation in CSMA/CA mode. This is achieved by making lower-priority access categories postpone contention when packets belonging to categories with higher priority are pending for transmission. For this purpose, specific slots in which stations advertise the priority of the current packets to be transmitted are allocated. However, they are only present after the occurrence of successful frame exchanges. Thus, in lightly loaded conditions as well as after channel errors or collisions, the priority resolution mechanism is not employed. In this work, we evaluate the implications of these features on the QoS experienced by each access category. Results show the network provides a complex performance behaviour caused by the interdependence of higherpriority traffic contention and lower-priority traffic preemption.