Teriyaki sauce sugar recrystalizing

I have a tried and true recipe for teriyaki sauce from the restaurant I work for back in the States, but I've come to Fiji to open a second location with my chef and our sauce keeps recrystalizing over night when we store it in the walk-in cooler, turning completely solid. We use raw sugar back home, but tried our first batch with local brown sugar and it turned quickly. We can remelt it but I don't want to keep cooking it again and again. We did our second batch with regular white sugar and cut the sugar by 1/2, but to no avail. We assumed it was the sugar content level or the molasses in the brown sugar but I can't find any information. Does anyone have any ideas, suggestions or explanations? Much thanks.