Maybe the problem is that they are running out of celebrities with something of interest in their family history. They must pre-research dozens of people in the hope of finding a handful that might have something to reveal that makes their story worthy of dedicating a whole programme to it or with enough twists and turns in it that it comes as a surprise to the celebrity. Which, in theory, is the point of the programme even though we all know that altruism doesn't play such a big part and that tv entertainment value is their prime interest.
While I find it interesting to an extent to nod a wink to the social history of the era these descendants lived in, I would prefer the programme time to be dedicating to finding another generation or branch, but then, if there isn't anything to be found, I suppose they have to fill in the minutes somehow.
And parts of each programme are obviously contrived to build up tension. The William Roach edition was a perfect example. They showed him a census record with, I believe, his gt grandfather having some hitherto unknown occupation and then sent him off to some historian to explain to him what it meant and give him the occupation he evolved to later on. Yeah, but if they had pulled up the next census record it would have shown us that anyway and we could have saved about 5 mins of airtime to explore something else. And I know that because while the programme was on, I found the next census record myself after wondering why they hadn't done so.