The solvent in the developing tank is only 1/2 cm high and the samples are spotted at 3 cm.
The spots should always be above the level of solvent. If the spots are below the level of solvent, then the sample spot will dissolve or wash off in the solvent much before traveling in the Thin layer chromatography plate. So the sample will not separate out and the purpose of Thin layer chromatography will not be fulfilled.
Hence separation of components of sample mixture will not happen.
The spots should always be above the solvent, so that when the solvent move up through capillary action, the solvent comes in contact with the sample mixture and carries it up. As different component travel with different speed , so separation takes place.