First ensure that the sample has been milled to pass a 0.5mm screen. Then ensure that the sample is vortexed sufficiently after step 3 and during step 4.
It is imperative that the sample is vortexed thoroughly at 2, 4 and 6 minutes during boiling. These steps aim to remove any sample from the sides of the test tube and homogenise your solution.
We suggest that you take a look at the video provided on the product page for best practice.