I've been a ham for over 40 years, but never has there been such a prolonged period of such poor propagation. This has been especially the case over the past six months or so, and they continue to deteriorate.
What could possibly be the cause?
This is taking into account information from sites such as Spaceweather and the sites linked to from this page on my site. Even when solar conditions seem to be favorable for good band conditions, the bands are terrible.
Sure, there's solar flares, CMEs, and other events that typically make the bands go "dead". But those effects have been taken into account.
NOT caused by increased man-made noise
In many locations, man-made noise is a factor. But I asked this question from the viewpoint of hams living in quiet areas; my location is almost noise-free, and there are times when my s-meter is zero even with the preamp on. This is about the signal levels dropping.
This image below is from K5BIZ's website. The network of receivers feeding data to it from K5BIZ's beacons is the most reliable indicator of real-time HF band conditions we've found. The majority of the time, the arrows on each band graph there have mostly been in the red areas of the the graphs for most of these bands like below, and they closely indicate what we hear on the bands. It wasn't always like this.
I know we are near the solar minimum, but it certainly seems that something else is at play. Many other hams concur. What could it be?