This is usually caused by using a
solution that isn't saturated. The cure is to dissolve more solute into the liquid. Stirring and applying heat can help to get solute into solution. Keep adding solute until you start to see some accumulate at the bottom of your container. Let it settle out of solution, then pour or siphon the solution off, being careful not to pick up undissolved solute. If you don't have any more solute to use, you can take some comfort in knowing that the solution will become more concentrated over time, as evaporation removes some of the
solvent. You can speed this process by increasing the temperature where your crystals are growing or by increasing air circulation. Remember, your solution should be loosely covered with a cloth or paper to prevent contamination, not sealed. See this link for details
http://chemistry.about.com/cs/growingcrystals/a/aa012804.htm
