Unfortunately, there are hundreds of possible causes of stuck and sluggish fermentations, and sometimes, even after careful investigation, we still can never understand what the cause may have been. With Mother Nature throwing us a different curve ball every year, add in the fact that the fruit always has a slightly different nutritional profile and you’re always making wine against a slightly different backdrop.
That being said, stuck and sluggish fermentations are most often caused by the following: Cold temperatures, inadequate yeast nutrition, microbial competition, unhealthy culture buildup/pitching technique and toxic ethanol levels towards the end of fermentation. Less often, they may be caused or exacerbated by low pH (high acid), pesticide residues, old yeast packets, opened/damaged yeast packets, too hot of a fermentation or high volatile acidity (VA) (often a byproduct of microbial competition). This is why it’s important to “check all the boxes” and do our best to mitigate any possible negative circumstances with our fermentations.
In your case, it’s strange you had two seemingly identical lots and one really stuck at higher sugar (1.021 SG, around 5.3 °Brix) and one almost finished (1.002 SG, around 0.52 °Brix). I would go through the above list and see if each lot was truly identical in its treatment. Was one lot in a colder spot in your cellar? Was one lot from part of the berry patch that may have had a pesticide accidentally sprayed on it? It’s so hard to tell.
The yeast you used, RC-212 can ferment up to around 16% alcohol so be sure to check your initial SG/Brix to make sure it would end up below that range once dry. You mention that both lots were a little slow to take off . . . RC-212 has what the manufacturer calls an “average” lag phase but is not more specific with how many days that may be. Be sure to download all information you can about your chosen yeast (usually called the “technical data sheet”) from the manufacturer’s website and follow directions to the letter. Specifically be sure that you scrutinize nutritional requirements and temperature requirements, and adjust your must accordingly to make sure final alcohol doesn’t go above the yeast’s maximum tolerance level.
As far as what to do to the wine now . . . you might want to try blending both lots together and see if you like the taste. If so, you could sterile filter, or fortify with spirits like brandy, and make a fortified dessert wine. With blueberries as your starting material, that might be a really nice taste option.
For the lot that is drier, I wouldn’t try to put it through a re-start. I find that when sugar is that low, the process of re-starting actually lowers the overall quality and you’re better off bottling slightly sweet (sterile filtered). However, let your taste be your guide. With your sweeter lot, I think you might want to give restarting a stuck fermentation a try. There is a good article by Jason Henrie on the procedure on the WineMaker website at https://winemakermag.com/article/704-troubleshoot-a-stuck-fermentation. In my winery, I use the basic procedure published by Enartis/Vinquiry on their website, www.enartisvinquiry.com. Click on “Technical Information,” then “Winemaking,” then “Restart a Stuck or Sluggish Fermentation.” They do a great job taking you through the procedure.