I have been brewing for years and I did not realize until just recently that liquid yeast packs (or vials) become increasingly ineffective as they grow older.
I was reading some articles the other day and one of them mentioned that if you buy a yeast pack that was produced on that very day you can only expect about 97% of the yeast cells in the pack to be viable. If the yeast is a month old it is only about 75% viable. If you are brewing a strong beer with old yeast (or even a somewhat strong beer) you are probably going to need either a second packet of yeast or you will need to make a yeast starter to build up a sufficient count of yeast cells to tackle the strong sugars in a high-gravity wort.
When I stopped at the home brewing supply store last night to pick up yeast for my next batch I paid attention to the production date on the yeast. I was a little bit annoyed that the newest packet in the style that i needed was a month and a half old. There were some packets that were almost 3 months old. It was a strain that was somewhat less common that other brewing yeasts, but still…
I have experienced under-attenuated batches of beer in the past, and this might be why.
In any case, there is a pretty handy calculator at MrMalty.com that helps you determine how many packets of yeast you need under different conditions.
Have you ever experienced any problems with brewing yeast?