How to dissolve white dextrin in water?
Try heating the water. 

Sugar is also very water soluble, but just dumping sugar into cold water is going to take forever to dissolve. You'll age before you get a clear solution. So, add heat and stir it, and it should dissolve.

PS: Some dextrins are only partially solvable. If sigma aldrich says that your particular dextrin is fully solvable, then it should be. Check the label.