I work on Humic acid to produce a high concentration of humic substances in water. In pH=9 or 10, it is dissolved by NaOH. However, in low temperature, the liquid becomes agglomeration like blood clotting.
Divalent cations also work on dissolution of humic acids. So avoiding or checking for those also would worth. Increasing pH seems also be effective. You can also change humic acid source like using smaller molecules which are richer in ionizable groups in basic pH.
Humic acid is insoluble in water at any PH. You can dissolve the acid in Basic solution such as NaOH and bring down the pH to required level by slowly adding acid solution. However, You cant get the pH bellow 2. Sonication will help you to dissolve without agglomeration.