Is your observation in the past couple days or in the past couple months? We did lose the large contribution from Cougtek for a couple reasons. One is that the bonus for large multi-processor CPU folding ended sometime in January if I remember correctly. The other reason is he had to repurpose those systems for some other task.
There have been other changes with the distribution of work units. I've noticed I get a fair amount of Core 15 WUs which offer no QRB. On those days my PPD is a good percentage lower than normal. I'm unclear if the developers have resolved issues with the Core 18 WUs under Maxwell GPUs which make them much less efficient. Last I checked that was still on-going and requires work from Nvidia but that was several weeks/months back.