Yes, the sample of sand could still be a heterogeneous mixture even if all the grains were the same size, because heterogeneity refers to the composition and not just the size of particles. If the sand grains are made up of different minerals or substances, the sample would still be considered heterogeneous due to the varied composition of the grains.

If the initial hypothesis is not supported, you can go back to the drawing board and hypothesize a new answer to the question and a new way to test it. If your hypothesis is supported, you might think of ways to refine your hypothesis and test those.