After adding a sachet of 71B yeast to a 50ml starter at about 40 °C, I've noticed absolutely no activity. The yeast sunk to the bottom of the container and just stayed there. This is the first time I experience this problem (at least I think it's a problem). Usually, the there will be a layer of froth and some minor bubbling in the container.
The only difference between this time and the 14 or so times I've done this before is that the yeast were frozen rather than refrigerated. According to a page at Lalvin's website, freezing their yeast for storage is acceptable.
I've already tried 3 sachets of frozen yeast and got the same odd result. I've just inoculated the last starter mixture because I thought I'm just being too paranoid. I hope I am, and I hope I'll see a thin layer of foam on top of my must in the next few days.