I've been worried that the new batch of hydromel I started last weekend hadn't been fermenting; the bubbler wasn't going at all, and, while there was sediment, and surface yeast, it was as much as the previous batch had given me.
A few days ago, I tried to re-pitch it by taking a sample from the hydromel I have resting, which didn't have much of an effect. Today, I noticed a small amount of bubbling, which was harder to see than I had expected because the water level in the bubbler was a little low, making the bubbles smaller.
I took a density measurement, and got 10.0 degrees Brix, barely lower than the 10.4 that I started with. So I decided to repitch again with a fresh packet of yeast. I started to shake the carboy to get the yeast distributed, and the bubbler started going like mad, shooting water up out of the top.
Apparently, it had been fermenting, but trapping all the gas in solution! Shaking it released all the gas at once, leading to the violent de-gassing.
Hopefully it keeps bubbling smoothly, but I'm definitely going to be more careful with these recipes in the future.