What is PDT ?
Low “useful” data traffic is one thing, but there might be a high amount of signalling data. What is the amount of RLC blocks transmitted for these cells, compared to other neighbor cells ? Is it less ?
Are you saying that within the same area (same BSC, same PCU), some cells are congested, and others are not, while the GPRS traffic (signalling + data) is approx. the same ?
In this case, it’s probably, either:
different GPRS parameters (MCS adaptation, PDCH/users, TBF/PDCH, etc.)
wrong load sharing of the PCU (is it something you configure ? in alcatel, that’s automatic)
or perhaps it’s just that the TCH (voice) traffic is just using up all radio resources, therefore the GPRS looks congested.
There are so many possible problems there… you really need to dig deeper in your QoS indicators 🙂