After the first few editions, I think they started getting into trouble with using too much recycled content (which was coming from the old PBS Tracks Ahead TV series). They've tried to improve the situation in the last DVD or two but seemed to err with the issuing an edition devoted only to prototype diesels, which a great many subscribers reported returning.
Admittedly, these DVD's are generally aimed at newbies and more experienced hobbyists seemed to have largely lost interest in them. While there has been talk of extending the series another dozen or so DVDs, a lot depends on how well the next few do in the marketplace. Volume #8 should be appearing any time now.
The latest one (9th DVD although not strictly in the DPB series) is "Realistic Layout Operations" so maybe they are trying to make up for the "Big Power" DVD described above.
They kept sending them to me even though I returned them. I can see why there is an issue. It was recycled and the price was out of touch for the content.
I've been in Model Railroading for about 35 years, and still enjoy the DPB DVDs. Some of the content is a bit simplistic, but all in all both my wife and I enjoy watching. Every time David Popp says "...I took and ..." we both chuckle.