I always have a little bit of undissolved powder in my solution when I mix it. However, they are all dissolved into the solution the next day. I use hot water as per spec for my mix.

I do not know this to be 100% true but makes sense: if you have undissolved powder when you prepare your tray, filter it out because if they get on the paper, they are highly active that emulsion may over develop on that spot.

Chances are, those powders are gone the next time you check your mix.