I did something very similar. I got about 2-3 weeks of good rise from a sponge starter made using a packet of yeast and stored in the fridge with periodic feedings (a few loaves, cinnamon rolls, and yeasted pancakes), then sometime in the 3rd week it lost its vigor (very little rise when fed). But it didn't go bad - the last thing I made with it had a slight sourness, but barely any rise. I took it out of the fridge, and have kept it on the counter with daily feedings, hoping to convert it to a sourdough starter (maybe the good bacteria had arrived, but the wild yeasts needed more time?). After a few days with little activity, it sprang to life in the last 24 hours! Perhaps conventional baker's yeast has a limit as to how many times it can regenerate before losing the ability to propagate?

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