DNS is a water-soluble molecule, but do note that sodium acetate has a fairly negative ΔG of hydration and can thus reduce the concentration of water available to otherwise dissolve other compounds (this property is often employed for nucleic acids). High concentrations of glucose would also be problematic for the solubility of some compounds. Have you tried diluting your sample prior to addition of your assay reagents?

For reducing sugar assay, the DNS solution contains NaOH which makes the reaction buffer highly alkaline.

I have exactly the same problem as you. Did you solve it?

Yes my problem got solved, avoid using buffer except for preparing the substrate, use only distilled water.