Brass molds are a real heat sink, much more so than aluminum or even iron. Even though you may have pre-heated your mold on a hot plate, the small 22 caliber boolits don't carry much heat with them to the mold as you cast. Think of it as pouring heat into the mold. As you cast, you may be losing a small bit of temperature each time you do a pour. Step up the cadence and pour large sprue even if it overflows is what I would try first. Just have a pan or something underneath to catch it. Increase the alloy temp as necessary to keep the mold hot enough. It's a balance, and you'll eventually find it.