My yeast is dying or something is stopping fermentation

Hello,
I'm looking forward to being here and learning better techniques on home brewing root beer. I'm on my 4th or 5th batch of home made root beer, but I've been having some trouble recently and was hoping to get advice. I make everything from scratch, use maple syrup (sometimes with barley malt syrup) as sweetener, and champagne yeast, fermented in amber glass w/ EZ Caps/clamps. I use a candy thermometer to make sure the brew is less than 90 degrees before adding it to the bottles.

The first few batches I added less than 1/16th tsp of yeast to the whole batch (2.5 Qt), before bottling. Fermented them for 3-4 days, before refrigeration. They pretty much all over-fizzed when opened.

I changed tactics on the next batch and added approximately 25 tiny grains of yeast to each bottle. Fermented for 48 hrs, before refrigeration. This gave me the best results. No over-fizzing at all. However, using this same technique of adding yeast to the bottles, on the 4th and 5th batch, they're all completely flat! No fermentation at all.

I've also used the same sanitation method for all batches: very hot water in the sink, with added distilled vinegar (about 2-3 Tablespoons). I want to avoid chemical sanitation or sterilization--but some of you might suggest otherwise. The vinegar method worked fine on the first few batches though.

Any suggestions?

How are you handling your yeast when counting out these 25 tiny grains and how is the remainder stored between batches? If you're using the same packet of yeast, it could be that it's not viable any more. I've re-used yeast from a packet that was opened and then frozen, but even then it loses its potency over time. Get some new yeast and try and ferment again and see what happens.

Makes complete sense. I use stainless steel spoon and a magnifying glass when counting out the yeast. I didn't think the yeast would diminish in potency as I've had it in the fridge since buying. However, that was at least 6 months ago. So, I'll head out to get some more yeast and try again. Thanks!