Hi Lizzie, a mature starter should be fairly resistant to being overtaken by mold our undesireable bacteria, so I wonder if your starters ever reached that level of maturity. With our sourdough starter recipe you want to see the starter doubling predictably with twice a day feedings. It's aroma should also be pleasant when the starter reaches maturity. This process typically takes at least 10 days, and it's often around the two week mark when the starter fully stabilizes. 

Once you've achieved a mature starter, you have the option to store it in the refrigerator when you're not planning to bake with it, and a mature starter can last quite a long time in the refrigerator. However, when it's in the refrigerator it's "surviving" rather than "thriving." It's when your starter is kept at room temperature and allowed to ferment through several feeding cycles that your starter builds the populations of wild yeast and friendly bacteria that keep it healthy and active and allow it to perform well in your baking. 

When you plan to store your starter in the refrigerator it's best to give it a feeding and then let it sit out at room temperature for 2-4 hours, or until it begins bubbling, before placing it in the refrigerator. 

Depending on how long it's been in the refrigerator it can take up to a few days of twice a day feedings at room temperature to restore it to its normal aroma and activity level. 

To prevent your starter from becoming sluggish you want to be sure that these room temperature feedings are spaced so that you're able to replenish (feed) your starter when it's at its highest point of rising, or just beginning to fall. This keeps the yeast active and vigorous. Allowing the starter to fall significantly between feedings tends to lead to sluggish yeast behavior. 

If need be you can adjust the ratio of ingredients in your starter feeding routine to slow down the ripening process so that your twice a day feedings line up closer to the peak. This might prove necessary if you live in a warm climate and you notice your starter is peaking after only a few hours. In this case, offering your starter a relatively larger meal can help slow down the process, since it will take longer for the starter organisms to consume and fement a relatively larger amount of flour and water. And don't worry, this "larger meal" doesn't have to mean using up more flour, since you can reduce the portion of starter relative to the water and flour. For example, our normal feeding routine consists of 113g each of starter, water, flour. This is considered a 1:1:1 ratio of starter:water:flour, by weight. To extend the ripening time for this starter you could consider trying a 1:4:4 ratio, with your twice a day feedings looking like this: 28g starter + 113g water + 113g flour. 

I hope this helps! 

Barb

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