Water Temperature Too Low
Powder detergents don't always dissolve well in low water temperatures. Although product manufacturers might claim that powder is suitable for cold and hot water wash loads, certain products take longer to dissolve in cold water. If you're running a short wash cycle on cold water and use a powder detergent, some of the powder might not completely dissolve during the wash cycle. Reduce the amount of powder you use when doing a small wash load at a low temperature to ensure the detergent dispels in the water.
Poor Water Quality
The water quality can impact the ability of the water and powder detergent to combine and create suds. In general, soft water usually requires less detergent than hard water for the same load size, so a slight adjustment in the amount of powder you use might improve dissolve issues. Furthermore, the minerals in the water can integrate poorly with the powder and limit its ability to completely dissolve. In this case, you might need to forgo powder detergent altogether or at least scale back on the amount that you use in each load.
Collects Inside Laundry
Powder detergent can sometimes get trapped inside rolled pant legs, pockets, pillow cases or even in socks when you pour it into the washing machine after the laundry is already inside. If any amount of powder gets caught inside laundry items, it might not have access to the water to dissolve. Put powder detergent into the washing machine first and turn the water on to give them time to incorporate before placing your laundry inside.
Liquid detergents provide the same cleaning power as most powder detergents. Consider switching to liquid detergent if you're having problems with powder not fully dissolving in your machine. Liquid detergent comes pre-dissolved, so you don't have to worry about a powder residue getting on laundry or collecting inside the washing machine as you sometimes to do with powder detergents.