There are a few general points that apply to all crystallization methods. It is important that the sample be as pure as possible. When crystallization attempts consistently yield oils, the sample is probably not pure. The solvents or cocrystallizing materials should be as pure as possible. Contaminants may often break down the desired sample.
New glassware is so smooth that there are no nucleation sites available on the exposed surfaces.
Also, new glassware from the manufacturer usually has a variety of dusty contaminants. If a sample only yields small crystals, the method should be altered so as to slow down the growth step. Slowing the crystal growth sometimes requires changing the method used to grow the crystals. Avoid vibrations near your growing crystals. Some methods work in a few hours, and other methods require weeks or even months for success.