What to do about a un-dissolved substance? To exterminate a coackroach infestation, I bought solid boric acid (in powder form). Rather than awaiting the cockroaches' encounter with the boric acid, I thought about spraying boric acid onto affected areas or cockroaches. I only took high school chemistry, but remembered solubility so checked its solubility in water. I then compared it with the solubility of other compounds, to see if this were an effective idea:

Boric acid: At 100 Celsius, 27.53 g/100 g of water.
Sodium chloride: At 100 Celsius, 38.99 g/100 g of water.

This looks fine, so I poured boiling water into a bottle with the boric acid powder. I shook it and saw the boric acid dissolve. Yet two weeks later, the boric acid seems to have reversed dissolution. Now, it's separated from the water and sits at the bottom of the water.
Can substances do this? What should I do now? I'd need a pot to boil the mixture over the stove. But then, in the interest of health and safety, I couldn't use this pot anymore, so don't want to sacrifice one.
Update: With thanks to and in view of the instructive answers, would it help to refill the bottle, to benefit from the low solubility of boric acid at room temperature? There's only around 10% of water left, with the boric acid sitting at the bottom.
I think what you forgot is that the solubility of boric acid (or sodium chloride.. or most solids) is lower at room temperature.

So when you dissolved the boric acid in boiling water, there's no problem at first. As it cooled, you will have a supersaturated solution until eventually the boric acid will recrystallize or crash out.

I'd create a new mixture using room temperature water and add only a small amount of boric acid (as mentioned by @K_P, about 5g / 100 mL of water). Let it dissolve slowly while you mix.

Then if you spray, you shouldn't leave large amounts of boric acid around.

Updated Answer

Yes, if there's only ~10% water, you will want to add more (room temperature) water. This will dissolve more boric acid. Stir/shake to dissolve as much as possible, wait a bit and then you can spray again.

As mentioned, you don't want to add water to a hot acid. In this case, adding more room temperature water to a room temperature acid will probably produce heat, but add slowly and carefully.