Water evaporating from your bottle is concentrating the
remainder, and some crystalizes out. To redissolve it, you have
to add more water. Just shaking won’t change the fact that your
bottle has a saturated solution in it which cannot hold more
material without having water added.
But don’t worry about it too much. It’s doing no harm. if you
dislike it, just make the stuff up more dilute. The main reason
we usually make it up as strong as we can is because when you
spray it on, the water is chilling the metal, and it’s less work
to spray less flux. but in many schools, where kids waste a lot
of it, they make it up substituting a gallon of water for the
quart one might use with 80/80/120 grams of each chemical. That
solution is then only a quarter as strong, and though it takes
more to properly firecoat a piece of silver, it still works just
fine. And it won’t crystalize out so fast.