You are making me thirsty! I am a huge fan of cider and perry (pear cider), especially when it’s tart, dry-ish and has a sparkling finish. Yummy yum yum! I applaud you for keeping this ancient and tasty art alive in your neck of the woods.
You present an interesting scenario above. Since I don’t know your batch sizes (volume) I can’t help you calculate what 5 pounds (2.3 kg) of dry sugar (cane sugar? Malt sugar?) would do to your specific gravity, though an initial specific gravity of 1.034, being about 8.5 °Brix, certainly needs some sugar added to it. It’s possible you added too much, which would account for your stuck fermentation of about 6.5 °Brix or so (the 1.025 that you mention above). Once alcohol levels get above 14.5%, most yeasts start to struggle to complete a fermentation, which is why I try to never start my fermentations above 25.0 °Brix (1.106 specific gravity) if I can help it. Though the number varies from year to year and winery to winery, in my experience, alcohol conversion rates tend to be 0.55–0.58 alcohol points for every degree of Brix you start with. I recommend making sure you don’t add too much sugar; be sure to measure your initial starting Brix and add sugar gradually to your must to avoid over doing it.
I’m no pomologist (fruit scientist), but from what I understand, letting pears soften (i.e. get ripe) prior to pressing doesn’t make the sugars more fermentable per se, it just makes more sugar. Soft, ripe fruit has a higher sugar concentration than hard, unripe fruit due to ethelyne gas triggering enzymatic activity, which transforms starches into sugars. My guess is that you experienced something this year that lead to both the stuck fermentations you witnessed.
Though the causes of stuck fermentations are sometimes difficult to deduce (I myself was stumped by one of my own this year — arrrgh!), here are some possibilities: you added too much sugar to your batches (and 5 lbs/2.23 kg of sugar could go a long way depending on your batch size), you had a bad batch of yeast that wasn’t robust enough to finish the fermentation, you had an organism get into your fermentations (either acetobacter or pediococcus) that caused the VA to climb and the yeast to give up, or you didn’t have enough of a certain kind of micronutrient.
The latter reason is particularly interesting because fermentations conducted with high levels of dry sugar (beet, cane, etc) often don’t have the required nutrients or healthy fermentation factors your average yeast beastie requires. Perhaps the year you started with riper fruit, your yeast enjoyed the goodies (amino acids, pantothenate, biotin, vitamins, nitrogen, not to mention suspended solids/pulp that are often important for fermentation kinetics) that come with fermenting real fruit juice rather than a white sugar solution. Pure sugar adds degrees Brix to a fermentation (higher potential alcohol), but little else