If your airlock stopped bubbling early then there’s possibly three major things that could’ve gone wrong (and dozens of little details)…
99% of your problems will be solved by focusing on the three main issues… the first and most likely reason for yeast to stop bubbling early is not pitching a good amount of healthy yeast into your wort…
If you are brewing with extract, then you should have enough nutrients in your wort for yeast, but if you are brewing all grain beers, then you may need to make sure your water source has enough calcium and other minerals vital to yeast…
Provided your wort has enough nutrients, the amount of yeast you pitch will determine how good of a fermentation you will have… this amount can vary depending on how healthy and strong the yeast is… if you are using brand new yeast out of a vial or smack pack, chances are the yeast is not very strong… strong yeast comes from yeast starters and left overs from previous fermentations…
But even if you underpitch you should still see fermentation unless your wort is devoid of nutrients… to find out if your beer is not fermenting check your gravity reading… if it’s constant, then chances are it’s not fermenting… but more than likely it will be…
The second reason and possibly the most common reason why brewers see their airlock bubbling stopping early is lack of temperature control…
When yeast are exposed to increasing temperature environments, they tend to become more active… if they are exposed to decreasing temperature environments, they tend to become less active… most brewers who see their airlock bubbles stop will usually notice it in the morning… after an overnight drop in ambient temperature…
… again if this happens, take a gravity reading and check to see if fermentation is still happening…
You may just have to warm up the fermentor and perhaps rouse the yeast to get it back in suspension (floating)…
If you’ve pitched a healthy amount of active yeast, and your temperature is not the issue, then you need to start looking into viability of your yeast… in other words, check to see your yeast is good…
If you are making yeast starters, this should be a problem because you’ll see at least a little bit of krausen form on top and you know you’ve got a good population… but if you are using left over yeast, then you need to figure out how many times the yeast has been reused and what percentage is good yeast…
You may need to wash your yeast to get rid of bad yeast… but the effort to do this is too much and it’s better to just start with a new vial of yeast…
The most important thing to keep in mind about airlock activity is that it doesn’t necessarily mean that fermentation isn’t happening… your gravity reading is the only thing that will tell you if fermentation is stuck…
