It is incredibly important to make sure you're using hot water when making hot chocolate. If the water is lukewarm, the hot chocolate will become lumpy and hard to dissolve even when stirred. 

Pro-Tip: Try adding a small amount of water to the hot chocolate powder to make a paste, mixing it well until all the lumps are dissolved before filling the cup up the rest of the way with hot water. As well, the product will mix more thoroughly if the cocoa mix is put in cup first before the water.