I dunno if shaking helps tho, becuase its a solid that sits at the bottom of the bottle, I know i shook mine like hell and it didnt dissolve, Its an old bottle tho, just see if there is a precipitate at the bottom of the bottle, it should look milky. Nothing u can do about it but now u will kno where its coming from. I never use PH down, if im ever low i just add more water to the res. the general rool for dwc is to have the water level at the point where the bubbles will pop and hit the medium/net pot. U want the better part of your net pot damp, this will keep the root ball happy.