My first attempt at a yeast starter was a failure. I didn’t know why at the time, but it was obvious the flat, non-bubbling mixture wasn’t any kind of “starter”- it was completely dead. I made it in an empty, cleaned 2L pop bottle so I could see through the bottle pretty easily. I followed the directions for size, amount of malt extract, etc. I was even frequently shaking it up in order to aerate.
The problem was I was suffocating it. I kept the lid closed tightly, thinking it needed to act as my sort of airlock barrier to keep all the gremlins out that they make you fear as they drill into your head about sanitizing while homebrewing.
However, by keeping the lid on tight, there were 2 problems. First was any CO2 created by the fermentation had nowhere to go. So it would just build up gas inside the bottle and build pressure. Eventually this would stifle out the fermentation reaction.
The second problem was I wasn’t introducing any new oxygen to the hungry yeast process. As I later learned, in the case of the yeast starter, an airlock doesn’t work. It solves problem #1 (pressurization and CO2 build-up), but doesn’t allow any extra oxygen supply to help keep the yeast propagation going.
So I learned the following routine with the lid was the best approach: 1) Keep the lid loose while the bottle/jug is sitting in a warm spot and fermenting, 2) Tighten the lid only when shaking it to aerate the wort (with the oxygen you’ve let in with the loose lid while it was sitting on a shelf. After following this process, I’ve had good luck with my starters fermenting and multiplying yeast.