Hi Holly, it sounds like  your starter just needs a little more TLC. Once your starter is rising predictably then it's important to feed twice a day at room temperature, which helps to keep the yeast active and vigorous. I would try increasing your feedings to twice a day at room temperature for a few days and see if your starter returns to a more predictably rising behavior (doubling in size within about 6-8 hours), with no more white on top. Also, check that your starter has an aroma that is not unpleasant. I would also take care to cover your starter a bit more securely, so it doesn't dry out on top. The reason we say to "cover loosely" isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the lid to pop off. As long as there is room above the starter level for the gases to accumulate, and you open your jar now and then to vent the gases, it's fine to use a lid, or cover your starter container securely with plastic wrap. You want to avoid developing a crust on top of the starter, which can inhibit the rise. 

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