Yeah, take a gravity reading before you assume nothing's happening. That's the only SURE way to tell you what's happening. Honestly unless you boiled or froze your yeast, in the 21st century, modern yeast just doesn't typically not work... what doesn't work is people not taking a gravity reading to know what's going on and thinking airlock bubbling or lack of it or what a beer doesn't or does look like can tell you what's REALLY going on.
Rather than just thinking you need to throw more yeast at it because you think nothing's happening, when it more than likely IS fermenting along just fine, use the one diagnostic tool, your hydrometer to know for sure. I've brewed for 10 years, thousands of gallons of beer, extract, all grain, liquid, dry, low gravity, high gravity, and not once, have I ever had yeast fail me...I've never had yeast not start... I've had plenty of airlocks not bubble...I've had plenty of big or tiny krausens, or fast krausens that dissapeared in a day, or krausens that stuck around for a week or more after I confirmed fermentation had stopped with gravity readings...but I've never ever had yeast not do their job....
Yeast are amazing and resilient creatures that can survive and singlemindedly do their jobs. In fact the current strain of yeast called "Cry Havoc" of Charlie Papazian sat for nearly 20 years in a fridge. He had a jar of infected yeast, that he put away to study later, and forgot about it, and it got piled in the back of his keezer or beer fridge, forgotten. 2 decades later he stumbled upon it, and for ****s and giggles made a starter with it, not expecting much. To his surprise it worked, and when he brewed a batch of beer with it, he even found that not only did it work, but 1) the wild yeast that caused the infection long died out 2) and it made for an extremely crisp and clean beer.
People have also cultured yeast that was trapped in amber for thousands of year. And recently grown yeast and fermented beer in space.
So that's why I always find it so amazing, that even despite making starters that prove the yeast is working, many new brewers can't seem to trust these amazing creatures.
The biggest problem, is that WE HUMANS think we're in charge, and we try to impose OUR timeframes on them. We expect the yeast to work when we want it to, and expect it to finish at the exact moment the instructions say it will. But as I say all the time, yeast can't read, they don't know what day it is, they don't know the time. They don't know we have a deadline (which we shouldn't) so if it doesn't act exactly how we think it will, even though we know the yeast is fine. We immediately think something's wrong.
When usually the only thing wrong is our lack of faith in them.
There's a tiny list of reasons why yeast could fail to start, and that mostly involves pitching your yeast into too hot wort, or freezing the yeast... that's pretty much it...
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